a 2 
(ə; āwhen stressed)
(ə; āwhen stressed)indef.art.
1. Used before nouns and noun phrases that denote a single but unspecified person or thing: a region; a person.
2. Used before terms that denote number, amount, quantity, or degree: only a few of the voters; a bit more rest; a little excited.
3.
a. Used before a proper name to denote a type or a member of a class: the wisdom of a Socrates.
b. Used before a mass noun to indicate a single type or example: a dry wine.
4. The same: birds of a feather.
5. Any: not a drop to drink.
[Middle English, variant of an, an; see AN1.]
Usage Note: In writing, the form a is used before a word beginning with a consonant sound, regardless of its spelling (a frog, a university, a euphemism). The form an is used before a word beginning with a vowel sound (an orange, an hour). · An was once a common variant before words beginning with h in which the first syllable was unstressed; thus 18th-century authors wrote either a historical or an historical but a history, not an history. This usage made sense in that people often did not pronounce the initial h in words such as historical and heroic, but by the late 19th century educated speakers usually gave their initial h's a huff, and the practice of writing an before such words began to die out. Nowadays it survives primarily before the word historical. One may also come across it in the phrases an hysterectomy or an hereditary trait. These usages are acceptable in formal writing.