To keep a thing out of a set deliberately. Exclusion is a judgment, and should be stated rather than silent.
Latin excludere, ex- (out) plus claudere (to shut); the verb moved from the physical shutting of a door against someone to the more abstract shutting-out of a thing from any set at all.
The deliberate keeping of a thing out of a set. Exclusion is a judgment, and a judgment made silently is still a judgment.