As a letter:
* The first letter of the alphabet.
* A symbol for a specific sound in language.
As a word:
* An indefinite article. This means it refers to any one of a group or class, like "a dog" or "a car."
* A shortened form of "one" or "any". For example, "a thousand dollars" or "give me a piece of cake."
In other contexts:
* A symbol for "one" in math.
* A grading term, sometimes representing "excellent" or "very good".
* A prefix in words like "asleep" or "ahead."
Can you give me some context? Knowing the situation where you saw "a" will help me understand what it means.