I have recently encountered the definition of an adjoint Lie group, i.e. a Lie group which is semisimple with trivial center. My question is, as you might guess from the title :-), why do we call such a group "adjoint"? In other instances where we use the adjective "adjoint", there exists a partner (think left/right adjoint functor) or it comes from another mathematical object (think adjoint representation "adjoined" to a Lie group).
Is there a comparable associated object to an adjoint Lie group, or how did this name come to be?