- (countable) An adult is a person who is of the legal age of majority.
- At age 18 years, he became an adult, so he could move out without his parents' consent.
- Adult usually means a person who is either age 18 years or older.
- An adult is a human who is mentally and sexually mature.
- The Harry Potter series was read mostly by young adults.
- An adult is a sexually mature nonhuman animal.
- Adult female cows feed milk to their offspring.