Compaction
Mechanically densifying soil or granular fill so it will not settle under load. Done in controlled lifts with plate compactors, jumping jacks, or rollers.
What does compaction mean?
Compaction is specified as a percentage of a laboratory maximum density, and it depends on moisture content as much as on effort, since soil too dry or too wet will not densify. Trying to compact a thick layer in one pass leaves the bottom loose regardless of how many passes are made on top. Trench backfill and utility crossings under slabs are the classic locations where compaction is skipped and settlement follows.
In practice
Cracks and dips that appear directly over a buried utility line are almost always a backfill compaction story.