Voluntary (adjective) — تطوعي أو اختياري: Done by choice; unpaid. Below are 7 words with a similar meaning; the linked ones have full GL Dictionary entries with their meaning in Arabic.
Given as an honour, without the usual requirements; unpaid.
optionalwillingfreeunpaidelectivediscretionary
Done outside the normal school or course timetable.
GL usage: arising out of a deliberate choice rather than chance or force.
Happening naturally without planning.