Uniqueness vs. Unicity

Uniqueness and unicity can be synonyms when they are used to describe something that is “unique”, meaning something that is distinct from all other things.

Are these terms interchangeable, or do they carry slightly different meanings?

• I suspect most people would take "Unicity" to be a town where everyone rides unicycles. Commented Feb 2, 2015 at 21:37
• You'll sound unique if you say unicity. Commented Feb 3, 2015 at 3:55

Mathematicians (whose native language is not English) may use unicity, modeled after the French unicité, in place of the preferable word uniqueness.

In a similar way, foreign mathematicians also use compacity in place of compactness. Even if the mathematician is a native speaker of English, if his teacher was foreign, he may still have picked up compacity and unicity.

Another one is analiticity. But there is no (common) English word analiticness, so in this case I guess there is no avoiding analiticity.

• (Upvoted.) Note, though, that this does not mean that mathematicians in general will understand what meaning you intent to convey when using unicity. In my (non-native English and non-native French) circle unicity will not be ring any bell. I'd even argue it's worse to use it in a mathematical context as keywords like "uniqueness" are used in a very codified manner and it's thus more confusing than the use in a non-specialized text. Commented Feb 2, 2015 at 22:25
• @Perseids "Unicity" is common enough in math that the MathWorld article Uniqueness Theorem begins "A theorem, also called a unicity theorem […]." It's still less common, but it's a competitive usage rather than being completely out of left field. I think GEdgar's answer captures that properly. Commented Feb 2, 2015 at 23:17
• independency vs independence too?
– BCLC
Commented Mar 19, 2018 at 8:48
• Also worth mentioning (the perhaps obvious) fact that France is a mathematics powerhouse. Commented Jan 6, 2020 at 9:49

All three of uniqueness, uniquity, and unicity are attested by the Oxford English Dictionary, each with several centuries of citations. There is no especial difference in meaning between them; there is, however, a staggering difference in frequency.

Here’s what you should therefore do:

• Use uniquity only when you wish to rhyme with such words as iniquity, or when you wish to express the antonym of ubiquity using a form closely parallel to that word.

• Use unicity only when you wish to rhyme with such words as complicity, or when you wish to express the antonym of multi-city. ¹

• Use uniqueness everywhere else.

If you don’t use uniqueness, you are apt to perplex readers for one of two reasons:

1. Some because they have limited vocabularies or imaginations and so waste time thinking you used an unknown word just because they didn’t know what it means and weren’t in the mood for looking it up.
2. Others because their vocabularies are broad enough that they recognize the synonym but instead waste time wondering why you have chosen a rare word instead of its workaday synonym.

What it all comes down to is that uniqueness is the only one in common use, as shown by this Google N-gram:

Click graph to go directly to full Ngram.

That means that uniquity and unicity are “special-effects” words only, with uniqueness doing all the real work to express “one-of-a-kindness” in the language.

Footnotes

1. Which does not in the second instance rhyme with complicity, since when opposing multicity, unicity has two stresses not one.

“We need a multicity solution to our traffic problems during rush hour. A unicity approach just won’t work.”

• @ChrisSunami All three words can be found in the OED. That doesn’t make them equally applicable.
– tchrist
Commented Feb 2, 2015 at 14:56
• Of course poets (who do rhyming poetry, nowadays a small minority) choose unusual words for the purpose of rhyming. Even Shakespeare did this on occasion. Commented Feb 2, 2015 at 15:04
• 'To suggest that as the main reason for using a word is worse than damning with faint praise.' Exactly. Commented Feb 2, 2015 at 16:04
• @ChrisSunami I'm not sure what you don't understand here. tchrist says that the words are attested (and thus valid descriptively and prescriptively). Then he recommends that you don't use them because they are very rare and many people will take exception to your use of them. Then he suggests a usage (poetry) where they will actually fit very well and nobody could possibly object. What's confusing or unclear here? Commented Feb 2, 2015 at 16:18
• I'm surprised Gilbert and Sullivan didn't write a song using them all in the same verse. Commented Feb 2, 2015 at 16:25

There are dictionaries which register more or less all words, whether they are "dictionary corpses" or not. The Longman Dictionary of Contemporary English has only unique and uniqueness, neither unicity nor uniquity. By the way, also the smaller Concise Oxford Dictionary has only uniqueness. I should say unicity and uniquity are dictionary corpses a journalist may dig out and revive to sweep his readers of their feet with his exquisite vocabulary or a poet may use these corpses when he is in need of a special rhyme.

PS I refer to my edition of The Concise Oxford Dictionary of Current English in bookform, 9th edition, 1995.

• oxforddictionaries.com/definition/english/unicity
– caub
Commented Feb 2, 2015 at 18:51
• @crl - oxforddictionaries.com should habe a note about the frequency of unicity. The British National Corpus has only one single instance for this word. For "uniqueness" there are 238. And for uniquity none. Commented Feb 2, 2015 at 19:05