If an engine failure occurs above the OEI ceiling, the aircraft has to descend ... in which case we need to be concerned with obstacles.
Generally, the two options are
(a) OEI net cruise capability will give at least 1000ft obstacle clearance .. ie we can clear the critical obstacle without too much angst.
(b) OEI ceiling is below the minimum level necessary for cruise obstacle clearance ... in which case the descent to lower levels has to account for obstacle clearance. The usual requirement is that OEI descent ("driftdown") has to clear all obstacles by 2000ft until the OEI net ceiling is reached .. when (a) becomes applicable.
If terrain considerations are critical the driftdown descent will be at max thrust and a speed appropriate to the particular aircraft model.
As to why the term "driftdown" is used, I have no idea .. perhaps one of the ancient ones may be able to tell us ... however, the profile at the start of the OEI descent will be comparatively steep and then flatten out progressively ... the final few thousand feet will be a very shallow descent .. so the term may just be descriptive of the final descent profile ...