When your friend says "a" to you online, they either mistyped their message or are saying "hey." The "a" greeting abbreviates the "ey" sound of "hey," which is especially popular among Fonzie and Canadians.
A is just one of many abbreviations used in slang, along with the common c, u, y, and r terms. While it cuts down on characters, it can confuse recipients, especially since "a" for "hey" is spectacularly obscure. If you do consider using a to say hi to someone, you should probably just pony up another letter and send "hi."
A, good to c u online again! How r u doing?
A sports statistic most often seen in basketball, hockey, and baseball to keep track of a player's assists.
In basketball and hockey, it is attributed to the player who helps a teammate score by passing the basketball or puck. In baseball, an assist is given to each fielder who touches or fields, even if it's unintentional, the ball before a putout.