Hello. Say we have matrix B. I understand that Tr(B) represents the sum of the diagonal (what is it called exactly? cant think of its word.. is it main diagonal im thinking of?)

Anyway what does Tr(B) actually mean? I know how to find it but what is it called? I used to think it meant transpose but after finding out it means the sum of the main diagonal it isn't transpose, so what is it? Just looking for a name