Bubble bath was a type of soap available for bathing, a luxury item for immersive baths.
History and specifics[]
In the year 2269 aboard the Federation starship USS Enterprise, Leonard McCoy attempted to synthesize a bubble bath soap for the visiting Katalya Tremain. (TOS novel: Vulcan!)
In 2285, when Carol Marcus began to suspect Vance Madison and Del March was misusing the Project Genesis computer storage allocations, she deflected having to inquire directly by instead jokingly asking if Del had been using her bubble bath again. (TOS - Duty, Honor, Redemption novelization: The Wrath of Khan)
In 2371, while exploring a marketplace in Vandorra in the galaxy's Delta Quadrant, Talaxian traveler Neelix mused that he was just about as happy as when he had taken an immersive bubble bath tub upon arriving aboard the Federation Starfleet starship Voyager. (VOY novel: Bless the Beasts)