a

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

• abbreviation acceleration
• abbreviation are (measurement)
• preposition In every; to each; per.
• 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.

from The Century Dictionary.

• 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).
• A modern provincial corruption of the pronoun I.
• 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 reduced form of of, now generally written o', as in man-o'-war, six o'clock, etc.
• An old (and modern provincial) corruption of have as an auxiliary verb, unaccented, and formerly also as a principal verb.
• All.
• The early form of ah, preserved, archaically, before a leader's or chieftain's name, as a war-cry (but now treated and pronounced as the indefinite article).
• A Latin preposition, meaning of, off, away from, etc.
• A prefix or an initial and generally inseparable particle. It is a relic of various Teutonic and classical particles, as follows:
• 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 of Anglo-Saxon of-, an intensive prefix, as in athirst, ahungered (which see).
• 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.
• The first letter in the English alphabet, as also generally in the other alphabets which, like the English, come ultimately from the Phenician.
• As a symbol, a denotes the first of an actual or possible series. Specifically
• In music, the name of the sixth note of the natural diatonic scale of C, or the first note of the relative minor scale; the la of Italian, French, and Spanish musicians.
• In the mnemonic words of logic, the universal affirmative proposition, as, all men are mortal.
• In mathematics: In algebra, a, b, c, etc., the first letters of the alphabet, stand for known quantities, while x, y, z, the last letters, stand for unknown quantities; in geometry, A, B, C, etc., are used to name points, lines, and figures.
• In abstract reasoning, suppositions, etc., A, B, C, etc., denote each a particular person or thing in relation to the others of a series or group.
• In writing and printing, a, b, c, etc., are used instead of or in addition to the Arabic figures in marking paragraphs or other divisions, or in making references.