Matrices along with good number of zero entries are called sparse matrices.
Refer the following matrices of Figure
Figure: (a) Triangular Matrix (b) Tridiagonal Matrix
A triangular matrix is a square matrix wherein all of the elements either above or below the main diagonal are zero. Triangular matrices are kind of sparse matrices. A tridiagonal matrix is also kind of square matrix wherein all of the elements except for the chief diagonal, diagonals on the instant upper & lower side are zeroes. Tridiagonal matrices are also a kind of sparse matrices.