structuring element

A small 0/1 array with a defined origin, slid across a binary image to define every morphological operation.

A structuring element $B$ is a small binary array with a designated origin cell (commonly its center) that is aligned with every pixel of a binary image $A$ in turn; every morphological operation -- erosion, dilation, and everything built from them -- is defined purely in terms of how $B$ 's footprint relates to $A$ 's foreground at that alignment.