Popcorn was a Human snack food made from cooked corn kernels, usually served with butter, cheese, and/or salt. Since the early 20th century, it was associated with films, as it was a traditional snack served at movie theaters.
On the Starfleet ship Enterprise NX-01, they had a "movie night" about once a week at which popcorn was usually served. (ENT: "Dear Doctor", "Horizon")
Travis Mayweather once noted when Commander Tucker accidentally "cloaked" his hand that it could be useful at movie night, if you have a date, to steal popcorn. (ENT: "The Communicator")
The crew of Enterprise also ate popcorn during a movie night in the mess hall in a nightmare experienced by T'Pol in 2153. (ENT: "Impulse")
While he was living in the Las Vegas Lounge holoprogram in 2375, Nog enjoyed eating popcorn while watching movies such as Shane. (DS9: "It's Only a Paper Moon")
In 2377, when Tom Paris recreated the Palace Theater, he also replicated some popcorn for his date with B'Elanna Torres. Later, when the crew was invited to a showing of Attack of the Lobster People, Neelix offered popcorn (buttered and unbuttered) as they entered. (VOY: "Repression") Paris later ate popcorn while watching cartoons in his quarters after recovering from being brainwashed by the Quarren. (VOY: "Workforce, Part II")