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$(N)) was N, at least for <=N<255; CHR$(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.