Definition:Rank/Matrix/Definition 2

Definition
Let $K$ be a field.

Let $\mathbf A$ be an $m \times n$ matrix over $K$.

Let $\mathbf A$ be converted to echelon form $\mathbf B$.

Let $\mathbf B$ have exactly $k$ non-zero rows.

Then the rank of $\mathbf A$, denoted $\map \rho {\mathbf A}$, is $k$.

Also see

 * Equivalence of Definitions of Rank of Matrix