Triangular matrix
In mathematics, a triangular matrix is a special kind of square matrix. A square matrix is called .mw-parser-output .vanchor>:target~.vanchor-text{background-color:#b1d2ff}lower triangular if all the entries above the main diagonal are zero. Similarly, a square matrix is called upper triangular if all the entries below the main diagonal are zero.
https://en.wikipedia.org/wiki/Triangular_matrix