King James Dictionary 
A is the first letter of the Alphabet in most of the known languages of the earth in the Ethiopic, however it is the thirteenth, and in the Runic the tenth. It is naturally the first letter, because it represents the first vocal sound naturally formed by the human organs being the sound uttered with a mere opening of the mouth without constraint, and without any effort to alter the natural position or configuration of the lips. The A has been proven to be the first natural vocal sound, and entitled to the first place in alphabets.
A has in English, three sounds the long or slender, as in place, fate the broad, as in wall, fall, which is shortened in salt, what and the open, as in father, glass, which is shortened in rather, fancy. Its primitive sound was probably aw. A is also an abbreviation used before words beginning with an articulation as a table, instead of an table, or one table. This is a modern change.
This letter serves as a prefix to many English words, as in asleep awake afoot aground agoing. In some cases, this is a contraction of Teutonic ge, as in asleep, aware, from the Saxon geslapan, to sleep, to beware. Sometimes it is a corruption of the Saxon on, as again from ongean , awake from onwacian to watch or wake. Before participles, it may be a contraction of the Celtic ag, the sign of the participle of the present tense as, ag-radh, saying a saying, a going. Or this may be a contraction of on, or what is equally probable, it may have proceeded from a mere accidental sound produced by negligent utterance. In some words, a may be a contraction of at, of, in, to, or an. In some words of Greek original, a is privative, giving to them a negative sense, as in anonymous.
Among the ancients, A was a numeral denoting 500, and with a dash A 5000. In the Julian Calendar, A is the first of the seven dominical letters.
Among logicians, A, as an abbreviation, stands for a universal affirmative proposition. A asserts E denies. Thus in barbara, a thrice repeated denotes so many of the propositions to be universal.
The Romans used A to signify a negative or dissent in giving their votes A standing for antiquo, I oppose or object to the proposed law. Opposed to this letter were U R, uti rogas, be it as you desire - the words used to express assent to a proposition. These letters were marked on wooden ballots, and each voter had an affirmative and a negative put into his hands, one of which at pleasure he gave as his vote, - In criminal trials, A stood for absolvo, I acquit, C for condemno, I condemn and N L for non liquet, it is not evident and the judges voted by ballots this marked. In inscriptions, A stands for Augustus or for ager, aiunt, , aurum, argentum, &c.
A is also used for anno, or ante as in Anno Domini, the year of our Lord anno mundi, the year of the world ante meridiem, before noon, and for arts, in artium magister, master of arts.
In algebra, a and first letters of the alphabet represent known quantities - the last letters are sometimes used to represent unknown quantities.
music, A is the nominal of the sixth note in the natural diatonic scale - called by Guido la. It is also the name of one of the two natural moods and it is the open note of the 2d string of the violin, by which the other strings are tuned and regulated.
In pharmacy, a or aa, abbreviations of the Greek ana, signify of each separately, or that the things mentioned should be taken in quantities of the same weight or measure.
In chimistry, A A A stand for amalgama, or amalgamation.
In commerce, A stands for accepted, as in case of a bill of exchange. Merchants also number their books by the letters - A,B,C, instead of figures. Public officers number their exhibits in the same manner as the document A, or B.
In mathematics, letters are used as representatives of numbers, lines, angles and quantities. In arguments, letters are substituted for persons, in cases supposed, or stated for illustration, as A contracts with B to deliver property to D. - In the English phraseology "a landlord as a hundred a year," " the sum amounted to ten dollars a man," a is merely the adjective one, and this mode of expression is idiomatic a hundred in a year ten dollars to a man.
Webster's Dictionary 
A, as a prefix to English words, is derived from various sources. (1) It frequently signifies on or in (from an, a forms of AS. on), denoting a state, as in afoot, on foot, abed, amiss, asleep, aground, aloft, away (AS. onweg), and analogically, ablaze, atremble, etc. (2) AS. of off, from, as in adown (AS. ofd/ne off the dun or hill). (3) AS. a- (Goth. us-, ur-, Ger. er-), usually giving an intensive force, and sometimes the sense of away, on, back, as in arise, abide, ago. (4) Old English y- or i- (corrupted from the AS. inseparable particle ge-, cognate with OHG. ga-, gi-, Goth. ga-), which, as a prefix, made no essential addition to the meaning, as in aware. (5) French a (L. ad to), as in abase, achieve. (6) L. a, ab, abs, from, as in avert. (7) Greek insep. prefix / without, or privative, not, as in abyss, atheist; akin to E. un-.
American Tract Society Bible Dictionary 
The first letter in almost all alphabets. In Hebrew, it is called aleph, in Greek, alpha, the last letter in the Greek alphabet being omega. Both the Hebrews and Greeks used their letters as numerals; and hence A (aleph or alpha) denoted one, or the first. So our Lord says, "I am Alpha and Omega, the beginning and the end, the first and the last;" thus declaring his eternity and that he is the cause and end of all things, Revelation 1:8,11 21:6 22:13 Isaiah 44:6 48:12 Colossians 1:15-18 .
Easton's Bible Dictionary 
Revelation 1:8,11 21:6 22:13 Hebrews 12:2 Isaiah 41:4 44:6 Revelation 1:11,17 2:8
Morrish Bible Dictionary 
Aleph, thefirstletter in the Hebrew alphabet. In numerals it stands for 1, and with two points for 1,000. A (alpha) the first letter in the Greek alphabet. The small letter with a dash after (α '), stands for 1. For this letter as a name of Christ see ALPHA.
Smith's Bible Dictionary 
A. See Alpha .