A minor, barely detectable earthquake, generally preceding a full-scale earthquake with approximately the same focus. Major quakes may follow a cluster of foreshocks by as little as a few seconds or as much as several weeks.
An earthquake that is smaller than, and precedes, a "mainshock". Foreshocks tend to occur in the same area as the mainshock. Foreshocks have not been observed before damaging earthquakes in British Columbia.
Any earthquake which is followed, within a short time span, by a larger earthquake in the exact same location can be labelled a "foreshock". (In the case of an earthquake swarm, this terminology is not generally applied.)