Dahermit,
It describes creep regardless of the engagement length. If you feel the engagement moving unevenly, you have creep. With a crisp trigger you shouldn't feel the engagement move at all before the sudden movement that lets the hammer go. With a rollover trigger you will feel the engagement move, but it will be totally even and smooth and will go right through the hammer drop and won't have any hesitations along the way. Obviously, the first stage of a two-stage trigger doesn't count since the take up of the first stage doesn't move the engagement.
When a person creeps around a location, their movement is starting, stopping to listen or to check for observers, starting again, stopping again, etcetera. It's just not not a smooth motion. Same applies to a trigger. That's where the term comes from.