How does 设A是..., 若A的..., 则称A是... work for mathematical definitions?
「設」is short for 假設 (Suppose that...)

「若」means if

「則稱」, literally then (it is) called, should just be taken as then.



設A是一個m×s矩陣，若A的任一行是集{1,...,n}的一個s-排列，任一列是集{1,...,n}的一個m-排列，則稱A是一個m×s拉丁矩。
「Suppose that」A is an m by s matrix.「If」every row of A is an s-permutation of {1, ..., n} and every column is an m-permutation of {1, ..., n},「then」A is an m by s Latin rectangle.