Hawaiian shirts were a style of collared dress shirt originating from the island chain of Hawaii on the planet Earth. They were extremely colorful, short-sleeved, buttoned-down shirts, usually – but not necessarily – with a prominent floral design.
They were worn when off duty, usually in the context of a real or simulated vacation. Trip Tucker threw on a Hawaiian shirt prior to disembarking for shore leave on Risa in 2152. (ENT: "Fallen Hero", "Two Days and Two Nights")
Similarly, Tom Paris was seen in several different Hawaiian shirts while on board USS Voyager. One such occasion was when the Voyager crew fêted at a luau. Harry Kim also wore a Hawaiian shirt in 2373, one with a pineapple motif. (VOY: "Alter Ego")