A lower triangular matrix having 0s along the diagonal as well as the upper portion, i.e., a matrix A = [a_(i j)] such that a_(i j) = 0 for i<=j. Written explicitly, L = [0 | 0 | ... | 0 a_21 | 0 | ... | 0 ⋮ | ⋮ | ⋱ | 0 a_(n1) | a_(n2) | ... | 0].

lower triangular matrix | strictly upper triangular matrix | triangular matrix