indefinite article before words beginning with vowels, 12c., from Old English an (with a long vowel) "one; lone," also used as a prefix meaning "single, lone" (as in anboren "only-begotten," anhorn "unicorn," anspræce "speaking as one"). See one for the divergence of that word from this. Also see a, of which this is the older, fuller form.

In other European languages, identity between the indefinite article and the word for "one" remains explicit (French un, German ein, etc.). Old English got by without indefinite articles: He was a good man in Old English was he wæs god man.

In texts of Shakespeare, etc., an as a word introducing a clause stating a condition or comparison conjunction is a reduced form of and in this now-archaic sense "if" (a usage first attested late 12c.), especially before it.

"not having the same properties in all directions," 1854; see an- (1) "not" + isotropic.

1880, "without an amnion" (of amphibians and fishes); see an- (1) "not, without" + amniotic.

"free from echo; tending to deaden sound," 1948, in electronics, from an- (1) "not" + echoic.

"of unequal measurement," 1850, perhaps based on German anisometrisch (by 1836); see an- (1) "not" + isometric.

"without a navel," 1742, from Latinized compound of Greek an- "without" (see an- (1)) + omphalos "navel" (see omphalos).

"absence of urination," 1838, medical Latin, from Greek an- "not, without" (see an- (1)) + ouron "urine" (see urine) + abstract noun ending -ia.

word-forming element meaning "unequal, not equal," from Greek anisos "not equal," from an- "not" (see an- (1)) + isos "equal to, the same as" (see iso-).

"diminishing the sexual appetite," 1823, from Greek anaphroditos "without sexual desire," or from an- (1) "not, without" + aphrodisiac. Related: Anaphrodisia; anaphroditic; anaphroditous.

"characterized by or causing lack of oxygen in tissues," 1920, medical Latin, from Greek an- "not, without" (see an- (1)) + first two letters of oxygen + -ic. Anoxia "oxygen deficiency" is attested from 1931.