Real Dictionary

unseat
verb: If a horse unseats its rider, it causes him or her to fall off.
verb: When people try to unseat a person who is in an important job or position, they try to remove him or her from that job or position.

unseat in American English
to throw or dislodge from a seat; specif., unhorse
transitive verb: to dislodge from a seat, esp. to throw from a saddle, as a rider; unhorse

unseat in British English
to throw or displace from a seat, saddle, etc