Definition:Hadamard Matrix/Definition 1

Definition
A Hadamard matrix $H$ is a square matrix such that:
 * $(1): \quad$ all the entries of $H$ are either $+1$ or $-1$
 * $(2): \quad$ all the rows of $H$ are pairwise orthogonal.

Also see

 * Equivalence of Definitions of Hadamard Matrix