entry

An entry is one number sitting in one cell of a matrix — exactly like a single number in one box of a spreadsheet. The matrix is the whole grid; an entry is one occupant of one slot.

Each entry has an address given by its row number and column number, in that order: the entry in row i and column j is written a_ij (read “a sub i j”). So a_23 means the number in the second row, third column. By long-standing convention the row index always comes first, the column index second.

Knowing every entry and its position completely determines the matrix. Many matrix operations are defined entry by entry — for example, two matrices are equal exactly when every corresponding entry matches.