It's not just a simple letter. The OED is quite thorough listing 14 separate entries for "A" or "a". Three prepositions, a verb, an adverb, a conjunction, a prefix, a suffix and others. The entry for "A" as the letter itself is also extensive. Listing a vast number of references and abbreviations and acronyms, I'll only quote less than a quarter of the entry here.
the first letter of the Roman Alphabet, and of its various subsequent modifications (as were its prototypes Alpha of the Greek, and Aleph of thePhoenician and old Hebrew); representing originally in English, as in Latin, the ‘low-back-wide’ vowel, formed with the widest opening of jaws, pharynx, and lips. The plural has been written aes, A's, As. from A to Z: see Z 3.
The sounds now represented by A are thus symbolized in this work:
|1. (||in name||(|
|2. (||in bare||(|
|3. (æ)||in man||(mæn)|
|4. (||in father||(|
|5. (||in water||(|
|6. (||in want||(|
1 and 2 are also commonly represented by the digraphs ai, ay, as in pain, pay, pair (pen, pe, p(r)); and 5 by au, aw, as in laud, law (ld, l). Ai, ay rarely represent a diphthong (a), as in ay, Isaiah (a, aza); au is a diphthong (a) only in foreign words.
In unaccented syllables these vowels are modified, and obscured; thus:
|7. (||in village, marriage||(|
|8. (||in separate, adj.||(|
|9. (||in lunar, a||(|
II. The letters of the alphabet, or some of them, are also used to indicate serial order and distinguish things in a series, as the notes of the musical scale, the ‘quires’ or sheets of a book, classes of ships, propositions in logic, quantities in algebra, points, and hence lines and figures in geometry. As the order is in some cases fixed, A or a has some specialized uses:
2. In Nautical language: see A1 below.
3. In Logic: a universal affirmative.
4. In Abstract reasoning, hypothetical argumentation, Law, etc. A means any one thing or person, B another, C a third, etc.; as, A becomes surety to B for C; C fails in his engagements, on which B, etc.
5. In Algebra: a, b, c, and other early letters of the alphabet are used to express known quantities, as x, y, z are to express the unknown.
6. Designating a first-class road.
7. Used of a type of blood.
8. Sociol. Designating the highest (orlowest) of a series of social classes; now spec. the higher managerial, administrative, or professional class; a member of this. AB (Sociol.): pl., the membership of the two highest social classes A and B; also in sing. and as adj.
9. Designating a range of international standard paper sizes (as A0, A1, A2, etc.), based on a proportion of 1:2, with each size in the series having half the surface area of the previous one (see quot. 1937). Cf. B II. 2(v), C II. 4.
2. A 1. Applied in Lloyd's Register to ships in first-class condition, as to hull and stores alike. ‘The character A denotes New ships, or Ships Renewed or Restored. The Stores of Vessels are designated by the figures 1 and 2; 1 signifying that the Vessel is well and sufficiently found.’Key to the Register. Added to the names of ships, as ‘the fast-sailing ship “Sea-breeze”, A 1 at Lloyd's’, or used attributively, ‘the splendid A 1 clipper-built ship “Miranda”.’
Hence, fig. (familiar and savouring of commercial phraseology), A 1, or in U.S. A No. 1, is used adjectively for ‘prime, first-class’.