short for Standard ML
"Standard ML is a safe, modular, strict, functional, polymorphic programming language with compile-time type checking and type inference, garbage collection, exception handling, immutable data types and updatable references, abstract data types, and parametric modules. It has efficient implementations and a formal definition with a proof of soundness. "
Two popular varieties of this are
SML/NJ (SML of New Jersey, by Bell Labs, aka Lucent)
Moscow ML (ML of Moscow, duh..), by Peter Sestoft at the Royal Veterinary and Agricultural University in Denmark