Strictly, it is just SQL. It doesn't stand for anything. The project was originally known as SEQUEL, a fun word play on QUEL (from the Ingress/Postgres camp), but trademark issues saw it later lose the vowels.

"Structured Query Language" is a so-called backronym. "Standard Query Language" fits the pattern as well. As does "Super Quick Lookup", if you want to have some fun with it.