1. The floor or desk area taken up by a piece of hardware.
2. [IBM] The audit trail (if any) left by a crashed program (often in plural, footprints). See also toeprint.
3. RAM footprint: The minimum amount of RAM which an OS or other program takes; this figure gives one an idea of how much will be left for other applications. How actively this RAM is used is another matter entirely. Recent tendencies to featuritis and software bloat can expand the RAM footprint of an OS to the point of making it nearly unusable in practice. [This problem is, thankfully, limited to operating systems so stupid that they don't do virtual memory -- ESR]