Quoting Stroustrup from his 2007 History of Programming Languages paper:
'Alex named his containers, iterators, and algorithm library
“the STL”. Usually, that’s considered an acronym for
“Standard Template Library”. However, the library existed
—with that name—long before it was any kind of standard
and most parts of the standard library rely on templates. Wits have suggested “STepanov and Lee” as an alternative explanation, but let’s give Alex the final word: “ ‘STL’ stands for ‘STL’ ”. As in many other cases, an acronym has taken on a life of its own.'
'Alex named his containers, iterators, and algorithm library “the STL”. Usually, that’s considered an acronym for “Standard Template Library”. However, the library existed —with that name—long before it was any kind of standard and most parts of the standard library rely on templates. Wits have suggested “STepanov and Lee” as an alternative explanation, but let’s give Alex the final word: “ ‘STL’ stands for ‘STL’ ”. As in many other cases, an acronym has taken on a life of its own.'
http://www.research.att.com/~bs/hopl-almost-final.pdf