In the English alphabet:
* The first letter of the alphabet: It represents the sound of the vowel "ah".
* A definite article: Used before singular countable nouns to indicate a specific or general noun. For example: "a cat", "a book".
In other contexts:
* Abbreviate: It can be used to abbreviate words, like "a.m." for "ante meridiem" (before noon).
* A symbol: In mathematics, "a" can represent a variable, a constant, or a coefficient. In chemistry, it can represent the element aluminum.
* A measurement: It can be used to represent areas, like "acres" or "ares".
To give you a more specific answer, please provide some context. What are you referring to?