a·dult [? dúlt, á dùlt] noun (plural a·dults) 1. fully grown life form: a fully mature person, animal, plant, or other form of life 2. somebody legally adult: somebody who has reached the age of legal majority, generally 18 years of age in the United States adjective 1. completely grown: fully developed and mature an adult male adult life 2. for somebody mature: involving, relating to, or meant for mature people adult education 3. unsuitable for children: considered unsuitable for young people because of pornography, violence, or sexually explicit language [Mid-16th century. < Latin adultus, past participle of adolescere (see adolescent)] -a·dult·hood, noun -a·dult·ness, noun