What is Steinberg group (K-theory)?

In algebraic K-theory, a field of mathematics, the Steinberg group St ⁡ {\displaystyle \operatorname {St} } of a ring A {\displaystyle A} is the universal central extension of the commutator subgroup of the stable general linear group of A {\displaystyle A}.

It is named after Robert Steinberg, and it is connected with lower K {\displaystyle K} -groups, notably K 2 {\displaystyle K_{2}} and K 3 {\displaystyle K_{3}}.