volunteer [volunteers](one who voluntarily offers themselves for service) noun [UK: ˌvɒ.lən.ˈtɪə(r)] [US: ˌvɑː.lən.ˈtɪr]I volunteered. = Me ofrecí de voluntario.
voluntarioadjective
voluntary(done, given, or acting of one's own free will) adjective [UK: ˈvɒ.lən.tri] [US: ˈvɑː.lən.te.ri]John did it voluntarily. = John lo hizo voluntariamente.
voluntarioso adjective
willing(ready to do something that is not a matter of course) adjective [UK: ˈwɪl.ɪŋ] [US: ˈwɪl.ɪŋ]
involuntary(without intention; unintentional) adjective [UK: ɪn.ˈvɒ.lən.tri] [US: ɪn.ˈvɑː.lən.ˌte.ri]Breathing is an involuntary bodily process. = La respiración es un proceso corporal involuntario.
unintentional(not intended or deliberate; inadvertent; unwitting) adjective [UK: ˌʌ.nɪn.ˈten.ʃnəl] [US: ˌʌ.ˌnɪn.ˈten.ʃə.nəl]The company claims that the delay was unintentional. = La empresa afirma que el retraso fue involuntario.