BASIC's name for the
inverse of the
CHR$ (or
CHR, depending on dialect -- this was
BASIC!) function.
ASC would take a
string (BASIC didn't have a character type!) and return the
ASCII code of its first
character.
So ASC(CHR$(N)) was N, at least for 0<=N<255; CHR$(ASC(S$)) was the first character of S$.
I've no idea what ASC("") was. Probably 0, but maybe -1 or an error message. Most likely it varied between dialects; there was no standard for BASIC worth adhering to.