Word definition: a

Defiintion of a:

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

Synonyms of a:

amp, ampere, angstrom, angstrom unit, antiophthalmic factor, axerophthol, deoxyadenosine monophosphate, group A, type A, vitamin A

Antonyms of a:

See Also:

abamp, abampere, alphabetic character, blood group, blood type, current unit, dehydroretinol, fat-soluble vitamin, letter, letter of the alphabet, mA, metric linear unit, micromicron, micromillimeter, micromillimetre, milliampere, millimicron, nanometer, nanometre, nm, nucleotide, picometer, picometre, retinol, Roman alphabet, vitamin A1, vitamin A2

Webster Dictionary (1913) for a:

\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.

\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.
\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.
\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.
\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.
\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.
\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.
Pertaining to, or characteristic of, the Creoles. -- n. A Creole.
Having two barrels; -- applied to a gun.
Colored with different tints; variegated; as, a party-colored flower. ``Parti-colored lambs.'' --Shak.