Extra space inserted into memory structures to achieve address alignment -or- extra space between the frame and the content of an HTML element -or- extra spaces or zeros when printing out values using formatting print commands like, in C, the printf*-family of functions.
Padding in memory structures is the insertion of additional bytes of unused space to make the internal fields of a structure align to desirable address boundaries.
Padding in HTML layout is extra space inserted between the content of the element and its border or frame. Padding is space added to the interior of the element; margin is space around the exterior of the element.
In arrays, padding is addition of extra 0's (zeros) in one or more dimension of the array in a symmetric, circular or replicating manner. This is important for signal and image processing, in the context of fft and convolution.