How is the word "qua" used?

It's from Latin, meaning "what" or "as." The entry at Dictionary.com gives this example:

"The work of art qua art can be judged by aesthetic criteria only."

The point (aside from sounding a little pretentious) would be to contrast "art qua art" with, say, "art as a commercial enterprise" - where the criteria would be whether a particular piece is salable.

It's also used as part of the phrase (also from Latin) "sine qua non," meaning something essential to something else ("money is sine qua non for an American political campaign").

Qua can usually be read as the word "as".

It is an occasionally useful (and rarely used) link-word in English. I was just reading about qua in Fowler's (incl. the 3rd ed by Burchfield) a couple of days ago. Says Fowler: "The real occasion for the use of qua occurs when a person or thing spoken of can be regarded from more than one point of view or as the holder of various coexistent functions, and a statement about him (or it) is to be limited to him in one of these aspects":

"Qua lover he must be condemned for doing what qua citizen he would be condemned for not doing."

Here, "the lover aspect is distinguished from another aspect in which he may be regarded. The two nouns (or pronouns) must be present, one denoting the person or thing in all aspects (he), and the other singling out one of his or its aspects (lover, or citizen)."

This was the only way in which Fowler preferred the word be used, but in fact (notes Burchfield), it's often used in other ways:

• between identical nouns ("X qua X") as an emphatic version of "as":

"The presence of actual words is apt to confuse any estimate of the evocative power of the music qua music."

"I don't think that 'Hard Times' is a particularly good novel qua novel, whatever it may be as a social document."

"James Kirkup's poem about Jesus … is … an indefensibly bad poem qua poem."

• And sometimes, merely as "as":

"It cannot, qua film, have the scope of a large book." [i.e., it cannot, as a film,…]

"Qua phonetician, de Saussure has no interest in making precise the notion of species."

"Dressed in an Armani suit and espadrilles, he plays a cop qua existential hero."

But the word can seem pretentious, so you may want to avoid it — it usually adds nothing over "as", anyway.

(Burchfield ends with: "And as to usage, as is often the better choice of word, qua word.")

Beware that qua are truly 3 heteronyms (ie: same spelling, but different meaning and pronunciation). So user 'Alex' 's answer above does NOT refer to the same qua; it only introduces 2 of the 3 heteronyms. (Here is a helpful Venn diagram that depicts the relationships between pronunciation, spelling, and meaning of words)

I exerpt, but don't quote fully, Wiktionary on quā. For ease of readability, I eschew the use of >.

In brief, and if I haven't erred, the meaning of 'as' (discussed above) matches Etymology 1.
The use in 'sine quâ non' matches Etymology 2.

Etymology 1 (of 3) Adverb declined from quī. [...] (not comparable) [...]

1. On which side, at or in which place, in what direction, where, by what way (qua...ea...)

2. as; in the capacity or character of

3. In so far as (eg ens qua ens ("being as being"))

4. In what way, how, by what method; to what degree or extent

Etymology 2 (of 3) {pronoun} [=] ablative feminine singular of quī [“who, which”]

Etymology 2 (of 3) {pronoun} [=] ablative feminine singular of quis

I first encountered qua (repeatedly) many years ago in a translation of Thomas Aquinas's Summa Theologica, who used the (Latin) word to convey a specific meaning involving the calling out of a particular inherent quality or identity of a given thing. The term seems to retain a special sense in analytical philosophical works today. Here's a discussion of qua in Richard Cross, The Metaphysics of the Incarnation: Thomas Aquinas to Duns Scotus (2002):

My term 'qua_r', is thus correctly understood as 'in virtue of'. This definition of reduplication is found most clearly in Scotus:

_{(8.10) 'Qua' ...properly denotes that that which follows it is the formal reason for the inherence of the predicate: such as 'a human being, qua white, or qua coloured, is seen'.}

...

...Think of white-Socrates. It is true of white-Socrates that he is coloured qua_r white, and it is false of him that he is coloured qua_r man; but this obviously does not entail that Socrates qua_r man is not coloured—if it did, Socrates would be (absurdly) both coloured and not coloured.

...

The speculative analysis is more interesting. Consider the following from Aquinas:

_{(8.11) Just as in human and corporeal matters those things which it can be called into doubt whether they belong to a whole or a part, we do not ascribe to the whole simply or without determination if they inhere in a part: for we do not say that an Ethiopian is white, but that he is white according to his teeth.}

Aquinas's point here is that the predicate 'white' is not true of the whole Ethiopian but only of his teeth. Using 'qua_sa' to pick out this sense of 'qua', Aquinas thus accepts the following definition of 'qua_sa:

(B) x qua_say is F = y is a part of x, and y is F.

'Qua_sa' is thus as Aquinas presents it a sign of synecdoche. It qualifies the subject by modifying the reference of the subject term.

The terms qua_r and qua_sa that Cross uses (above) in his analysis of the formal logic of Aquinas and Duns Scotus are thus special subcategories of the more general reduplicative sense of qua (as "in virtue of") that he ascribes to Scotus near the outset of the extract.

Evidently one way in which qua continues to be used today is by modern scholars in connection with analyses and discussions of the logic of medieval philosophers.