Forced movement is any movement that does not require the unit to take their own step.
For example, is forced movement, since the moved unit doesn't step.
A unit being carried by another unit, e.g. , is also forced movement for the carried unit.
Effects that use the words drag, pull, or push are also forced movement, e.g. or .
Teleportation is forced movement. However, Teleportation effects can cross regional boundaries unless otherwise specified in their text.
Effects that require a unit to take a step are not forced movement. For example, is not forced movement, because the unit is taking their own step (and an Immobile unit wouldn't move at all). Confusingly, some lure effects on older cards use a phrase like "force an enemy to take a step towards," e.g. . This is still not forced movement, since the unit is taking their own step, and an Immobile unit wouldn't move; "force" in these cases is being used in the normal English usage.
The guiding principle for forced movement is whether or not the unit is taking their own step. If they aren't taking their own step, then it's forced movement!
Unless specified by card text, forced movement cannot cause a card to change regions (except for teleportation, which can change regions by default).