Sparse matrix - Wikipedia

An important special type of sparse matrices is band matrix, defined as follows. The lower bandwidth of a matrix A is the smallest number p such that the entry ai,j vanishes whenever i > j + p. Similarly, the upper bandwidth is the smallest number p such that ai,j = 0 whenever i < j − p (Golub & Van Loan 1996, §1.2.1). For example, a tridiagonal matrix has lower bandwidth 1 and upper bandwidth 1.

[shiumachi]

"a sparse matrix is a matrix populated primarily with zeros "疎行列

[gologo13]

疎な行列に対する専用の演算方法とかあるあしい