a

from The American Heritage® Dictionary of the English Language, 5th Edition.

• article Used before nouns and noun phrases that denote a single but unspecified person or thing.
• article Used before terms that denote number, amount, quantity, or degree.
• article Used before a proper name to denote a type or a member of a class.
• article Used before a mass noun to indicate a single type or example.
• article The same.
• article Any.
• auxiliary verb Have.
• noun The first letter of the modern English alphabet.
• noun Any of the speech sounds represented by the letter a.
• noun The first in a series.
• noun Something shaped like the letter A.
• noun The best or highest in quality or rank.
• noun The sixth tone in the scale of C major or the first tone in the relative minor scale.
• noun A key or scale in which A is the tonic.
• noun A written or printed note representing this tone.
• noun A string, key, or pipe tuned to the pitch of this tone.
• noun One of the four major blood groups in the ABO system. Individuals with this blood group have the A antigen on the surface of their red blood cells, and the anti-B antibody in their blood serum.
• idiom (from A to Z) Completely; thoroughly.
• preposition In every; to each; per.
• abbreviation acceleration
• abbreviation are (measurement)

from The Century Dictionary.

• An old (and modern provincial) corruption of all genders and both numbers of the third personal pronoun, he, she, it, they. So quotha, that is, quoth he.
• A Latin preposition, meaning of, off, away from, etc.
• A reduced form of the preposition on, formerly common in all the uses of on, but now restricted to certain constructions in which the preposition is more or less disguised, being usually written as one word with the following noun.
• Of place: On, in, upon, unto, into; the preposition and the following noun being usually written as one word, sometimes with, but commonly without, a hyphen, and regarded as an adverb or a predicate adjective, but best treated as a prepositional phrase. Similarly
• Of state: On, in, etc.: as, to be alive
• to be asleep
• to set afire; to be afloat; to set adrift.
• Of time: On, in, at, by, etc., remaining in some colloquial expressions: as, to stay out a nights (often written o' nights); to go fishing a Sunday; now a days (generally written nowadays). , ,
• Of process: In course of, with a verbal noun in -ing, taken passively: as, the house is a building; “while the ark was a preparing”(1 Pet. iii. 20); while these things were a doing. The prepositional use is clearly seen in the alternative construction with in: as, “Forty and six years was this temple in building,” John ii. 20.
• Of action: In, to, into; with a verbal noun in -ing, taken actively.
• A prefix, being a reduced form of Anglo-Saxon of, prep., English off, from, as in adown (which see), or of later English of, as in anew, afresh, akin, etc. (which see).
• A prefix, being a reduced form (in Middle English, etc.) of Latin ab-, as in abate (which see). In a few verbs this a- has taken a Latin semblance, as in abs-tain (treated as ab-stain), as-soil. See these words.
• A prefix, being a reduced form (in Latin, and so in English, etc.) of the Latin prefix ab-, from, as in avert (which see).
• A quasi-prefix, a mere opening syllable, in avast, where a-, however, represents historically Dutch houd in the original Dutch expression houd vast = English hold fast.
• An unmeaning syllable, used in old ballads and songs to fill out a line.
• The form of an used before consonants and words beginning with a consonant-sound: as, a man, a woman, a year, a union, a eulogy, a oneness, a hope. An, however, was formerly often used before the sounds of h and initial long u and eu even in accented syllables (as, an hospital, an union), and is still retained by some before those sounds in unaccented syllables (as, an historian, an united whole, an euphonious sound).
• An old (and modern provincial) corruption of have as an auxiliary verb, unaccented, and formerly also as a principal verb.
• All.
• An unaccented inseparable prefix of verbs, and of nouns and adjectives thence derived, originally implying motion away, but in earlier English merely intensive, or, as in modern English, without assignable force, as in abide, abode, arise, awake, ago = agone, etc.
• An apparent prefix, properly a preposition, the same as a, preposition
• A prefix, in ado, originally at do, northern English infinitive, equivalent to English to do. See ado.
• A prefix of Greek origin, called alpha privative, the same as English un-, meaning not, without, -less, used not only in words taken directly or through Latin from the Greek, as abyss, adamant, acatalectic, etc., but also as a naturalized English prefix in new formations, as achromatic, asexual, etc., especially in scientific terms, English or New Latin, as Apteryx, Asiphonata, etc.
• A prefix of Greek origin, occurring unfelt in English acolyte, adelphous, etc.
• A prefix of Greek origin, occurring unfelt in atlas, amaurosis, etc.
• A modern provincial corruption of the pronoun I.
• A reduced form of of, now generally written o', as in man-o'-war, six o'clock, etc.
• A prefix or an initial and generally inseparable particle. It is a relic of various Teutonic and classical particles, as follows: