I fail to see what is so difficult about that definition. Indentation is logical spacing. Alignment is the visual spacing.

TidyPerl knows the difference as shown above.

Or, to use a heretical example, Python forces you to use only indentation, and forbids alignment.

I don't think it gets much simpler than saying "The stuff you like to be 2 spaces wide but the guy sitting next to you likes to be 4 spaces wide". That's the indentation.