voluntary

Done, given, or acting of one's own free will; not compulsory; unpaid.

• volunteer/ˌvɒlənˈtɪə/(n/v)志愿者；自愿做
• voluntarily/ˌvɒlənˈteərɪli/(adv)自愿地

• voluntary sector
• voluntary basis
• voluntary agreement
• purely voluntary
• voluntary redundancy
• voluntary contribution

1. 1.Participation in the scheme is entirely voluntary — no employee will face consequences for choosing not to take part.
2. 2.The organisation relies heavily on the voluntary sector to deliver services that the state no longer provides directly.

• optional/ˈɒpʃənəl/— not required
• discretionary/dɪˈskreʃənəri/— at one's own discretion
• self-initiated/selfɪˈnɪʃieɪtɪd/— begun by one's own decision

from Latin "voluntarius," from "voluntas" (will)