Commonly thought of as an abbreviation for "Structured Query Language", and sometimes as a recursive abbreviation for "SQL: Query Language". In fact, it is not an acronym or abbreviation at all.

Officially, SQL is the name given to a language composed of a standard set of human readable commands for accessing and manipulating records in a relational database. -- It is a relational database language. Amongst other things the language consists of statements to select, insert, update, delete, query and protect data.

Note (regarding pronunciation and usage): Because of a quirk in the early history of SQL development many people incorrectly pronounce it "sequel". According to the SQL standard, the official pronunciation is "ess-cue-ell". In light of this, the following statements are examples of incorrect and correct usage respectively:

• "a SQL database" Wrong (precede 'ess' with "an")
• "an SQL database" Right!
  

Of course, there will always be a few who haven't read the standard and others who believe standards are for chumps. That's ok too. The official standard pronunciation is provided here in the belief that it is best to at least know the rules before breaking them. Also, you should know that "Sequel" is the name of a software company in the U.K., and an old one in the U.S. so using that pronunciation instead of SQL may cause some understandable confusion among some industry people.