An n x n
square with the elements from a
set
S of size n occurring exactly once in each
row and column. The set S is usually {0, ..., n-1}
in papers concerning them, as any element
can then be referred to as (row + column) modulo n (in the case of the group table for the cyclic group of order n).
The study of Latin squares is a branch of combinatorics
and people like to write about critical sets and
mutually orthogonal Latin squares.