Meaning of “volunteer” - Learner’s Dictionary

volunteer

verb uk us /ˌvɒlənˈtɪər/
OFFER [ I, T ]

to offer to do something without being asked or told to do it:

[ + to do sth ] Rob volunteered to look after the kids.
ARMY [ I ]

to join the army, navy, etc without being officially told to join:

In 1939 he volunteered for active service.
INFORMATION [ T ]

to give information without being asked:

No one volunteered the truth.

(Definition of “volunteer verb” from the Cambridge Learner’s Dictionary © Cambridge University Press)