What is the precise definition of a non-square matrix in echelon form? Say we have something like:

1, 2, 3, 4, 5, 6 | 7

2, 3, 4, 5, 6 ,7 | 8

3, 4 ,5, 6, 7, 8 | 9

How would the echelon form of that look like? Don't worry about explaining that set of equations having so and so amount of parameters and infinite solutions. I'm just wondering how the echelon form of a non-square matrix is defined.

Thx in advance.