Seizure
A meaningful interference with a possessory interest in property, or a restraint on a person's freedom of movement.
Seizures of persons range from brief investigatory stops (Terry — reasonable, articulable suspicion) to arrests (probable cause). Seizures of property require probable cause that the item is contraband or evidence. The difference between a consensual encounter, a stop, and an arrest is graded by how much a reasonable person would feel free to leave.