noun
principal diagonal; main diagonal
In linear algebra, the diagonal of a square matrix that runs from the top-left to the bottom-right.
正方行列の主対角線上の要素の和をトレースという。
The sum of the elements on the principal diagonal of a square matrix is called the trace.
対角線 is a general diagonal line, while 主対角線 specifically refers to the principal diagonal in a matrix.
Compound of 主 (principal) + 対角線 (diagonal line).