"Space boomer" or simply "boomer" was 22nd century Earth slang for a person who had been born in space and spent most of their lives there.
Crews of Earth Cargo Service freighters, such as the ECS Fortunate and the ECS Horizon, were considered boomers. (ENT: "Fortunate Son")
Prior to the launch of Enterprise NX-01, the Humans with the most space experience were boomers, so a boomer, Travis Mayweather, was selected to be a bridge officer and helmsman on that ship's historic mission. (ENT: "Broken Bow") Mayweather was also nicknamed "resident boomer" by his crewmates. (ENT: "Fortunate Son")
After Mayweather expressed his inability to think of a place as home unless it had warp nacelles attached, Tucker showed disdain for his opinion, thinking it typical of boomers. (ENT: "Strange New World")
Mayweather always suspected that boomers were more affected than anyone else by lost colonies, since those colonists also had chosen to live a life "off-world". (ENT: "Terra Nova")
Background information[]
Rick Berman reckoned that the concept of boomers was thought up by Brannon Braga. ("In Conversation: Rick Berman and Brannon Braga", ENT Season 1 Blu-ray special feature) Braga himself once characterized the concept of boomers as "an interesting idea" but remarked, "It was badly executed." ("To Boldly Go: Launching Enterprise, Part III: First Flight", ENT Season 1 Blu-ray special feature) He elaborated, "There are some things that you think are cool when you first create a show, like the space boomer idea. It didn't really go anywhere. I think we did one space boomer episode. But what sounded good on paper, born and raised in space, never been to Earth, what's their perspective? When everyone gets on a ship, week to week, it's kind of like: 'Who cares what the boomer has to say?'" Berman implied that he agreed, also recalling, "I thought this was a great idea." ("In Conversation: Rick Berman and Brannon Braga", ENT Season 1 Blu-ray special feature)
In the writers' second draft script of "Breaking the Ice", Archer asked Vulcan Captain Vanik if he had actually never previously encountered a boomer ship, to which Vanik implied that was correct. In the final version of "Breaking the Ice", the possibility of a boomer vehicle having been encountered by Vanik is ruled out by T'Pol simply commenting, aboard Enterprise, "I believe this is Captain Vanik's first visit aboard an Earth vessel."
In an ultimately unused line of dialogue from the first draft script of "Fortunate Son", Trip Tucker responded to Travis Mayweather, who had explained that boomer culture allowed for lots of time for procreation between ports, by remarking, "Sounds like there's some definite advantages to being a Space Boomer." The working title for "Fortunate Son" was "Untitled Boomer Story".