optional; of your own free will or design; done by choice; not forced or compelled
The voluntary donation program raised awareness and funds for community causes.
Vocabulary like voluntary appears on the Digital SAT in Words-in-Context questions, where you choose the word that fits a passage's meaning and tone. You can also drill this word with vocabulary flashcards.
superstar; someone who is dazzlingly skilled in any field
jelly-like; having a relatively high resistance to flow
clasp; a holding device attached to a workbench
possible to see; capable of being seen; or open to easy view
hateful; marked by harshly abusive criticism
capable of exploding; liable to lead to sudden change or violence
talkative; marked by a ready flow of speech
ample; large in volume or bulk