Placentia, city (1990 pop. 41,259), Orange co., S Calif.; inc. 1926. Once a rural farming community, it has become a residential city with manufactures that include industrial products, foods, furniture, construction materials, electronic equipment, machinery, and plastics. Placentia has grown along with the continued development of the Orange co. area. Abundant orange trees exist throughout the city. Many residents work in the nearby aerospace and high-technology industries.