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Is there an adjective that means "every two days", i.e. is to a day as biennial is to a year?

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In regular conversation, the phrase is simply every other day. Technically, however, one could use bidiurnal. It appears the word may have been coined by Ursula M. Cowgill in her 1965 paper, A bidiurnal cycle in the feeding habit of Perodicticus potto, from which I quote thus (emphases mine):

A definite bidiurnal cycle exists; the data corrected for natural loss are shown in Figure 1. The χ2 for the 48-hr periodicity is highly significant (χ2 = 188).
Cowgill, PNAS, 420 (1965)

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    I'd be a bit careful about actually using "bidiurnal", given that it's a neologism with no real currency, that might easily be misunderstood to mean "twice a day". Interestingly, medical prescriptions often use QOD, which seems to be a bowlderisation of QD (Latin "quaque die" - every day) with the O standing for "Other" added in the middle. Apr 23 '11 at 17:10
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    @Tim: Bidaily is not a formal coinage; biannual strictly means twice a year, while biennial means once every two years.
    – Jimi Oke
    Apr 23 '11 at 17:15
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    @Jimi: My point was that bidiurnal is an undesirable word both because it has no currency and because of the scope for misunderstanding. I fail to see how the existence of similar words also prone to misinterpretation or genuine ambiguity strengthens the case for supporting this wretched neologism. Apr 23 '11 at 17:19
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    @Fumble: I'm not here to argue about the desirability of the word ;) Tim asked if there is/was a [single-word] adjective that means every two days, and I simply provided an answer—a word that was used in a duly vetted scientific paper. And note that I suggested every other day as the standard phrase. Come to think of it, I actually like bidiurnal. We'll probably forever disagree on this :) I would hold, however, that your disapproval of the word, based on its perceived ambiguity, is unfair and unfounded, as bimonthly and biweekly have the same problem!
    – Jimi Oke
    Apr 23 '11 at 17:32
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    @Jimi: Fair enough. You managed to find the word, so undeniably it does exist in some sense, even if it's not defined in any dictionaries or recorded in the vast Google NGram corpus. There's no point in arguing about that - my point is simply that I wouldn't use it, and by implication I'd recommend others to avoid it. In the end, if people want to, they will use it regardless of my opinion. Apr 23 '11 at 18:10
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You could use "alternate".

I used to post on Instagram daily, but now I've switched to posting on alternate days. P.S Correct me if I'm wrong. 😅

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The abbreviation QOD or QAD (from Latin mean Quaque Alternis Die") means 'every other day' or 'every two days'.

Modern style in medical situations recommends using using the spelled out English 'every other day' because the Latin abbreviations are often misread.

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The Merriam-Webster Collegiate Dictionary, eleventh edition, defines 'tertian' as 'recurring at approximately 48-hour intervals -- used of malaria'.

It could cause confusion, though. Since Latin 'tertius' means 'third', you would think it should mean 'every third day', and some dictionaries simply define it that way. But that's because the Romans included the current day in naming intervals. One of the dictionaries cited at https://www.definitions.net/definition/Tertian clarifies it by defining the noun 'tertian' thus: 'a disease, especially an intermittent fever, which returns every third day, reckoning inclusively, or in which the intermission lasts one day'. My italics.

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  • The American Heritage Dictionary of the English Language, fifth edition (2010) offers this reconciliation of the "every 48 hours" meaning and the "every third day" meaning of tertian: "Recurring every other day or, when considered inclusively, every third day: a tertian fever." That is, the fever recurs on the third day, counting the initial occurrence of the fever as happening on the first day of the sequence.
    – Sven Yargs
    Mar 26 at 5:13
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To solve your problem, though not to answer your question directly, for phrases of the type:

A definite bidiurnal cycle exists...

it is perfectly natural formal English to say

A definite two-day cycle exists...

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