The recreation room (often abbreviated to "rec room"), and also known as a "crew lounge", was an area of a starship where crewmembers could go to enjoy games and other activities when off duty. They typically contained food synthesizers.
Constitution-class starships, such as the USS Enterprise, had several recreation rooms. Recreation rooms 3 and 6 were located on deck three. Another recreation room 3 was located on deck five. (TOS: "Charlie X", "Let That Be Your Last Battlefield")
A recreation room on deck five was also the place where Captain Kirk came to eat a chicken sandwich and drink coffee, in 2268, only to discover tribbles had also gotten into the food delivery machinery. (TOS: "The Trouble with Tribbles", DS9: "Trials and Tribble-ations")
At least one type of recreation room was capable of holodeck-like simulation of environments. By 2270, the Enterprise was outfitted with such a room in area 39 of the ship. This rec room was capable of creating pre-programmed illusions using holographic scenes, simulated weather including wind and temperature, and sounds played from audio tapes. The scenes it created included an 18th century European hedge maze, forests, blizzards, and a beach with seagulls. Even though the scenery extended beyond the room, one would still be able to walk up to and touch the walls during a simulation. (TAS: "The Practical Joker")
In TOS: "Charlie X", there is an off-screen area where some laughing crewmembers stand, their silhouettes cast on a nearby wall that is shown in the episode. The episode's final draft script made it clear that the off-screen area was recreation room 6.