1. [n]  the 1st letter of the Roman alphabet
2. [n]  the blood group whose red cells carry the A antigen
3. [n]  the basic unit of electric current adopted under the System International d'Unites; "a typical household circuit carries 15 to 50 amps"
4. [n]  a metric unit of length equal to one ten billionth of a meter (or 0.0001 micron); used to specify wavelengths of electromagnetic radiation
5. [n]  one of the four nucleotides used in building DNA; all four nucleotides have a common phosphate group and a sugar (ribose)
6. [n]  any of several fat-soluble vitamins essential for normal vision; prevents night blindness or inflammation or dryness of the eyes

A- is a 2 letter word that starts with A.

1. \A\ (named [=a] in the English, and most commonly ["a] in
   other languages).
   The first letter of the English and of many other alphabets.
   The capital A of the alphabets of Middle and Western Europe,
   as also the small letter (a), besides the forms in Italic,
   black letter, etc., are all descended from the old Latin A,
   which was borrowed from the Greek {Alpha}, of the same form;
   and this was made from the first letter (?) of the
   Ph[oe]nician alphabet, the equivalent of the Hebrew Aleph,
   and itself from the Egyptian origin. The Aleph was a
   consonant letter, with a guttural breath sound that was not
   an element of Greek articulation; and the Greeks took it to
   represent their vowel Alpha with the ["a] sound, the
   Ph[oe]nician alphabet having no vowel symbols. This letter,
   in English, is used for several different vowel sounds. See
   Guide to pronunciation, [sect][sect] 43-74. The regular long
   a, as in fate, etc., is a comparatively modern sound, and has
   taken the place of what, till about the early part of the
   17th century, was a sound of the quality of ["a] (as in far).
   2. (Mus.) The name of the sixth tone in the model major scale
      (that in C), or the first tone of the minor scale, which
      is named after it the scale in A minor. The second string
      of the violin is tuned to the A in the treble staff. -- A
      sharp (A[sharp]) is the name of a musical tone
      intermediate between A and B. -- A flat (A[flat]) is the
      name of a tone intermediate between A and G.
   {A per se} (L. per se by itself), one pre["e]minent; a
      nonesuch. [Obs.]
            O fair Creseide, the flower and A per se Of Troy and
            Greece.                               --Chaucer.
   

2. \A\ ([.a] emph. [=a]).
   1. [Shortened form of an. AS. [=a]n one. See {One}.] An
      adjective, commonly called the indefinite article, and
      signifying one or any, but less emphatically. ``At a
      birth''; ``In a word''; ``At a blow''. --Shak.
   Note: It is placed before nouns of the singular number
         denoting an individual object, or a quality
         individualized, before collective nouns, and also
         before plural nouns when the adjective few or the
         phrase great many or good many is interposed; as, a
         dog, a house, a man; a color; a sweetness; a hundred, a
         fleet, a regiment; a few persons, a great many days. It
         is used for an, for the sake of euphony, before words
         beginning with a consonant sound [for exception of
         certain words beginning with h, see {An}]; as, a table,
         a woman, a year, a unit, a eulogy, a ewe, a oneness,
         such a one, etc. Formally an was used both before
         vowels and consonants.
   2. [Originally the preposition a (an, on).] In each; to or
      for each; as, ``twenty leagues a day'', ``a hundred pounds
      a year'', ``a dollar a yard'', etc.
   

3. \A\ ([.a]), prep. [Abbreviated form of an (AS. on). See {On}.]
   1. In; on; at; by. [Obs.] ``A God's name.'' ``Torn a
      pieces.'' ``Stand a tiptoe.'' ``A Sundays'' --Shak. ``Wit
      that men have now a days.'' --Chaucer. ``Set them a
      work.'' --Robynson (More's Utopia).
   2. In process of; in the act of; into; to; -- used with
      verbal substantives in -ing which begin with a consonant.
      This is a shortened form of the preposition an (which was
      used before the vowel sound); as in a hunting, a building,
      a begging. ``Jacob, when he was a dying'' --Heb. xi. 21.
      ``We'll a birding together.'' `` It was a doing.'' --Shak.
      ``He burst out a laughing.'' --Macaulay.
   Note: The hyphen may be used to connect a with the verbal
         substantive (as, a-hunting, a-building) or the words
         may be written separately. This form of expression is
         now for the most part obsolete, the a being omitted and
         the verbal substantive treated as a participle.
   

4. \A\ [From AS. of off, from. See {Of}.]
   Of. [Obs.] ``The name of John a Gaunt.'' ``What time a day is
   it ?'' --Shak. ``It's six a clock.'' --B. Jonson.
   

5. \A\
   A barbarous corruption of have, of he, and sometimes of it
   and of they. ``So would I a done'' ``A brushes his hat.''
   --Shak.
   

6. \A\
   An expletive, void of sense, to fill up the meter
         A merry heart goes all the day, Your sad tires in a
         mile-a.                                  --Shak.
   

7. \A-\
   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[=u]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 [alpha] without, or privative, not, as in
   abyss, atheist; akin to E. un-.
   Note: Besides these, there are other sources from which the
         prefix a takes its origin.
   

8. Pertaining to, or characteristic of, the Creoles. -- n. A
   Creole.
   

9. Having two barrels; -- applied to a gun.
   

10. Colored with different tints; variegated; as, a party-colored
    flower. ``Parti-colored lambs.'' --Shak.
    

1. A prefix that means "without."
2. An enzyme that catalyzes the addition of adenine residues to the 3' end of pre-messenger RNAs to form the poly(A) tail.

Alpha, the first letter of the Greek alphabet, as Omega is the last. These letters occur in the text of Rev. 1:8,11; 21:6; 22:13, and are represented by "Alpha" and "Omega" respectively (omitted in R.V., 1:11). They mean "the first and last." (Comp. Heb. 12:2; Isa. 41:4; 44:6; Rev. 1:11,17; 2:8.) In the symbols of the early Christian Church these two letters are frequently combined with the cross or with Christ's monogram to denote his divinity.