To choose to participate in something

In a partial or full vacuum. / (figuratively) Out of context.

(medicine, neurology) A rare, severe physical disorder in which a person cannot speak and is paralyzed—with the possible exception of some voluntary eye movement—while their mental faculties remain fully intact.

(idiomatic) In the process of doing something considered criminal or otherwise wrong; used to emphasize the eye-witness evidence.

(computing) A copy of all or part of a disk drive retained for backup or replication purposes.

(databases, IBM) A backup copy of a database in a format that may be used, along with incremental logs, to recreate the database after a hardware failure

(idiomatic) To attend a class, educational presentation, meeting, etc. for one time, as a visitor. / (music) To take part in a group musical performance, especially a jazz session, as a temporary guest.

(insurance) Health insurance for a pet.

(Canada, chiefly politics, idiomatic) An occasion when a momentous career decision is made, especially a decision to resign or retire.

Face to face. / Despite.