singular matrix
Technical term in linear algebra for a square matrix that does not have an inverse; its determinant is zero.
A singular matrix does not have an inverse matrix.
If this determinant is zero, it is a singular matrix.
正則行列 (regular matrix) is the opposite: a square matrix that has an inverse and a non-zero determinant.
Compound of 非 (hi, 'non-') + 正則 (seisoku, 'regular') + 行列 (gyōretsu, 'matrix'). The term is a direct translation of the English 'singular matrix' in mathematical contexts.