The power of defendants in some state civil suits to move the case to federal court. Generally, if the case could have been brought in federal court (ie there is federal question jurisdiction or diversity jurisdiction) a defendant may demand that the case be moved, or "removed," to federal court. 28 USC 1441.
There are a few areas where, despite fitting in the above general rule, removal is not allowed. See 28 USC 1441(b), 28 USC 1445. In addition there are a few areas of law where removal is allowed even if the above condition is not met. See 28 USC 1442, 28 USC 1443