a·dult (?-d?lt?, ?d??lt)n.1. One who has attained maturity or legal age.2. Biology A fully grown, mature organism.adj.1. Fully developed and mature.2. Relating to, intended for, or befitting adults: adult education.3. Containing or dealing in explicitly sexual material; pornographic: adult movies.[From Latin adultus, past participle of adol?scere, to grow up; see adolescent.]a·dult?hood? n.a·dult?ness n.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.adult (?æd?lt; ??d?lt) adj1. having reached maturity; fully developed2. of or intended for mature people: adult education. 3. regarded as suitable only for adults, because of being pornographic: adult films and magazines. n4. a person who has attained maturity; a grownup5. (Biology) a mature fully grown animal or plant6. (Law) law a person who has attained the age of legal majority (18 years for most purposes). Compare infant[C16: from Latin adultus, from adol?scere to grow up, from al?scere to grow, from al?re to feed, nourish] ?adulthood nCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014a?dult (??d?lt, ?æd ?lt) adj. 1. having attained full size and strength; grown up; mature. 2. of, pertaining to, or befitting adults. 3. a. intended only for adults; not suitable for children. b. pornographic. n. 4. a person who is fully grown or developed or of age. 5. a person who has attained the legal age of majority. 6. a full-grown animal or plant. [1525?35