A shower is an area where one bathes under a spray of water.
Showers were present on Earth Starfleet vessels during the mid-22nd century. In 2151, Captain Jonathan Archer was showering in his quarters on Enterprise NX-01 when the artificial gravity went offline, sending him and the water droplets "floating" in the air. (ENT: "Unexpected")
By 2272, Starfleet vessels had begun to replace traditional, water-based showers with sonic showers. This was the case right up until at least the late-24th century. (Star Trek: The Motion Picture; TNG: "The Naked Now"; DS9: "If Wishes Were Horses"; VOY: "Juggernaut", "Live Fast and Prosper")
In 2370, Ensign Melora Pazlar explained to Doctor Julian Bashir that experiencing low gravity was "like slipping into a hot tub at the end of the day", to which Bashir joked, "I'm a shower man myself". (DS9: "Melora")