A valid convolution is a type of convolution operation that does not use any padding on the input.

For an $n\times n$ input matrix and an $f\times f$ filter, a valid convolution will return an output matrix of dimensions

$$\lfloor \frac{n-f}{s}+1\rfloor \times \lfloor \frac{n-f}{s}+1\rfloor$$

where $s$ is the stride length of the convolution.

This is in contrast to a same convolution, which pads the $n\times n$ input matrix such that the output matrix is also $n\times n$.