The Type 18 shuttlepod was a small short-range Starfleet craft carried by the USS Defiant in its first years, seen when Odo and Kira escaped the boarded ship in 2371.
The Type 18 is the Defiant's original auxiliary craft, a stubby pod with room for a handful of people. It is seen in the two-part Deep Space Nine story The Search, the ship's first mission to the Gamma Quadrant.
When Jem'Hadar soldiers board the Defiant, Odo carries the unconscious Kira to a shuttlepod and gets away. Odo, drawn by an instinct he cannot explain, has her set course for the Omarion Nebula, where they land on a rogue planet and find his people, the Founders. The pod remains on the surface while Odo learns what the changelings really are.
The type was little used afterwards, and from The Sound of Her Voice the Defiant is shown carrying the larger Type 10 shuttle instead.