This Counter32 counts packets that were never transmitted at all because they hit 16 collision attempts and were given up on. It represents an outright transmission failure rather than a delay, so a nonzero and growing value means real data loss on that port. Seeing txExcessiveCollision climb on a port serving a critical device like a rooftop radio would prompt an admin to check for a duplex mismatch or an overloaded shared segment on that link.