I'm looking for unusual/uncommon words that refer to a period of time. Something like fortnight:

(chiefly UK, Ireland, Australia, New Zealand, Singapore, dated in North America) A period of 2 weeks. - Wiktionary

But for various different amounts of time like a year, x number of years, x number of weeks, x number of days etc.

Note: Fortnight is used in British English but it is uncommon/archaic in US English. Please consider it as an uncommon time-period word example for this question.

New contributor
Christina Sims is a new contributor to this site. Take care in asking for clarification, commenting, and answering. Check out our Code of Conduct.
  • 3
    Which specific amount of time? Sep 22 at 21:03
  • 9
    "unusual" is pretty subjective. Why exactly is fortnight unusual to you?
    – Ivo
    Sep 23 at 12:51
  • 5
    Wikipedia has a whole article on these. Actually multiple articles... Sep 23 at 14:32
  • 6
    I’m voting to close this question because it is too broad as it will result in a list of all possible terms.
    – Greybeard
    Sep 24 at 9:24
  • 6
    @SamuelMuldoon Fortnight is a very common word where I live (England).
    – Showsni
    Sep 24 at 11:18

9 Answers 9


Try sennight when you are speaking of a week. I know of no similarly archaic word for fortnight.

in British English, Noun
an archaic word for "week"

The word makes sense when compared with "fortnight"

The word derives from the Old English term fēowertīene niht, meaning "fourteen nights" (or "fourteen days," since the Anglo-Saxons counted by nights).

Correspondingly, sennight refers to seven nights, hence a week. So a fortnight is two sennights.

  • 3
    Hebdomad is another word for a period of 7 days, related to the French hebdomadaire (weekly).
    – Stuart F
    Sep 23 at 9:46
  • @StuartF Excellent suggestion. I knew there was another word in my brain, but could I find it? No! Why not make it an answer?
    – Anton
    Sep 23 at 15:48
  • Heptad is a synonym of hebdomad also. It originally means "a group of seven" from Greek ἑπτάς, ἑπταδ- (heptas, heptad-), ἑπτά 'seven', but also used as "a group of 7 days, a week". Septuary is another rare word for things relating to the number seven, and also means "a week", from Latin septem 'seven'.
    – ermanen
    Sep 24 at 12:37
  • 1
    These are excellent terms. They are, of course, simple contractions: sennight is a shortened form of 'seven-nights', and fortnight is a shortened form of 'fourteen-nights'.
    – Ed999
    2 days ago

One unusual word is lustrum:

a period of five years. (OxfordL)

Wikipedia explains:

A lūstrum ([ˈluːs̠t̪rʊ̃ˑ], plural lūstra) was a term for a five-year period in Ancient Rome. The lustration was originally a sacrifice for expiation and purification offered by one of the censors in the name of the Roman people at the close of the taking of the census. The sacrifice was often in the form of an animal sacrifice, known as a suovetaurilia.

These censuses were taken at five-year intervals, thus a lūstrum came to refer to the five-year inter-census period.


When the tablets containing the vows to be offered for the welfare of the state during the next lustrum were handed to him, he left the duty of reciting them to Tiberius, saying that he would not take vows which he was never destined to perform. (YourDict)

  • Man that font + bold + italics really made me think it said "bustrum." Sep 23 at 21:41
  • 1
    In the Hollywood movie True Grit, a Cowboy picture starring John Wayne, made in 1969 but set in the 19th century, the character he plays uses the term lustrum to mean exactly this: a term of public office lasting 5 years.
    – Ed999
    2 days ago
  • @Ed999 John Wayne also manages other Latin quite often --> Posse --> Posse Comitatus - a legal right to copt a body of men (usually) to assist a 'Marshall'. 2 days ago

A nychthemeron is a day and a night, or a period of 24 hours.


For a period of x number of years; here are some unusual, archaic, uncommon, rare or obsolete (†) words from OED, usually directly coming from their Latin etymons and sometimes with ancient Greek roots (like pentad):

  • Three (3) years: triennium, triennial

  • Four (4) years: quadrennium, quadrennial, quinquenniad, tetraëterid

  • Five (5) years: quinquennium, quinquennial, †quinquennie, pentad

  • Six (6) years: sexennium, sexennate

  • Seven (7) years: septenary, septennary, septenniad (esp. in human life), †septimane (also means 'a week' from Latin septimana)

  • Eight (8) years: †octennial (Surprisingly, OED doesn't list octennium as an English word, which is also the Latin etymon of octennial)

  • Eleven (11) years: hendecad

  • Twelve (12) years: duodecad, duodecade, †duodenary

  • Twenty (20) years: vicennium

  • Three hundred (300) years: tricentenary, tercentenary

  • A thousand (1000) years: millenary, chiliad, †milliad

  • Two thousand (2000) years: bimillenary (OED adds that "for which bimillennium or some derivative of it would be the proper term")

  • One billion (thousand million) (1000000000) years: aeon, eon (it is used in Geology and Astronomy; but it also has the meaning "an age of the universe, an immeasurable period of time")

These findings show that one can form a word for a period of any number of years from their Latin version (obviously more applicable for round numbers when needed). For example, one can form novennium for a period of nine (9) years from Latin novennium, from Latin novem 'nine' + annus 'year' + -ium (which is not listed in OED and which you can find usages of).

  • 4
    You might like to add "sesquicentenary" and "semisesquicentenary" to your list, for 150 and 75 years respectively. Perhaps also "semiquincentenary" for 250 years. Sep 23 at 14:08
  • @PhilMJones Thank you for the addition. Per OED, "sesquicentenary" was only used for the 150th anniversary but not the time period; although one can use it for the time period too. I believe you've neologized "semisesquicentenary" and "semiquincentenary", clever. Per the last paragraph in my answer, there are many possibilities.
    – ermanen
    Sep 24 at 12:24
  • Eon has to be used in the plural, eons, in order to mean 'an immeasurable period of time' (in the sense of an unmeasured period).
    – Ed999
    2 days ago
  • @ermanen - I saw "semisesquicentenary" in print in 2007, and was so enamoured of the word that I used it in my father's 75th birthday card later the same year. I found "semiquincentenary" while searching online for "sesquicentenary". So while they are admittedly rather neologised words, they are not of my own invention. 2 days ago

A Moon could be used to refer to a lunar month, also known as a synodic month. The length is roughly twenty-nine and a half days. Another less-known unit is the sidereal day, which is approximately 23 hours and 56 minutes long; there are 366 of them in a normal year.

An Olympiad is a period of 4 years, the first Olympiad of the modern era comprising the Gregorian years 1896 to 1899.

  • The calendar of ancient Greeks was indeed organised into olympiads, but is there any real-life example of olympiad being used that way in the modern era?
    – jsw29
    Sep 24 at 16:06
  • No, the correct term for a lunar month, in Astronomy, is a lunation.
    – Ed999
    2 days ago
  • @jsw, the official title of the Olympic Games in 1948 (for example) was the "Games of the XIV Olympiad". I'm not sure if it gets use outside that context though; Chess Olympiads are international competitions that are not separated by 4 years.
    – Peter
    2 days ago

Of course, there is the "millifortnight" that is 1000th of a fortnight. It's approximately 20 minutes. Occasionally used in the aerospace industry.

New contributor
Wad6x is a new contributor to this site. Take care in asking for clarification, commenting, and answering. Check out our Code of Conduct.
  • 1
    That is used more widely than just the aerospace industry.
    – Chenmunka
    Sep 24 at 7:42
  • 1
    A milli-Helen --> A face that will launch a single ship. 2 days ago

From Wikipedia's Unit of time article:


  • Svedberg (100 femtoseconds)
  • shake (10 nanoseconds, from "a shake of a lamb's tail"
  • jiffy (1/50 or 1/60 s; one cycle of mains frequency)


  • milliday, or ".beat" (1/1000 day)
  • quarantine (40 days)
  • semester (126 days; wildly variable in university sense)


  • olympiad (4 yr)
  • indiction (15 yr)
  • jubilee (50 yr)
New contributor
David McKee is a new contributor to this site. Take care in asking for clarification, commenting, and answering. Check out our Code of Conduct.
  • 6
    The word "semester" derives from the Latin for "six months". The number of teaching days in an academic semester can vary widely.
    – Peter
    Sep 23 at 13:57
  • No, in physics a jiffy is the period of time that light (electromagnetic radiation) takes to travel a distance of one millionth of a millionth of a millimetre. That means that there are approx three hundred thousand billion billion jiffys in a second.
    – Ed999
    2 days ago
  • @Ed999 The unit is functionally meaningless without context. The quoted value in the anser is more properly a frame in NTSC or PAL (which was sometimes called a jiffy). In (some) physics contexts it is indeed the time it takes light to travel one femtometer. In a computer science context, it’s the length of one tick of a specific clock signal (usually the system clock) and only makes sense in the context of a specific platform (and I’m pretty sure this is where the NTSC/PAL usage came from). 2 days ago
  • Shake is "One Shake of a lamb's tail" --> nuclear bomb engineering 2 days ago
  • A debt jubilee: 7*7=49 years
  • A blue moon: roughly once every 3 years. (Nowadays "Once in a blue moon" means: "Not very often", and not "About every 3 years")


Coon's age means a very long time. It is an Americanism that has fallen out of favor and is considered offensive by many people. Coon is slang for raccoon, coined in the mid-1700s. The term coon's age was first used in the early 1800s and in fact, owes its origin to the folk belief that raccoons are long-lived.


New contributor
blah is a new contributor to this site. Take care in asking for clarification, commenting, and answering. Check out our Code of Conduct.
  • Blue Moon is second full-moon in a calendar month. 2 days ago

Not the answer you're looking for? Browse other questions tagged or ask your own question.