Popcorn is corn heated until the kernels burst into white puffs. A bag of popcorn, sweet or salted, popcorn at the cinema.
It is uncountable, so a popcorn is wrong — say a bag or a piece. The corn is a specific variety with a hard hull and a starchy inside; ordinary sweetcorn will not do it. Note the fixed association with cinemas, which is strong enough that popcorn movie means an undemanding entertaining film, and grab the popcorn means an argument is about to become entertaining to watch.
1nounpopcornCorn heated until it bursts into light white puffs.
“There was popcorn all over the floor by the interval.”
popcorn is a snack food.
Short for popped corn. by surface analysis, pop + corn.
Etymology from Wiktionary, CC BY-SA