Saucer section
The saucer section is a common feature of Federation starships. It is a large, disc-shaped section of the ship that usually houses the ship's bridge and crew quarters, separating them from the ship's engineering areas, which are usually housed in a cylindrical hull component.
The saucer section of a Galaxy-class starship is often described as the primary hull (with the engineering section referred to as the "secondary hull"). In this class, the saucer can separate from the engineering section and operate independently, albeit with much less power, since it doesn't have its own warp core.
Not all Federation ships have a Engineering Hull. There are several ship design, namely the Miranda and the Nebula classes, that are made up of mostly just a saucer with Warp engines attached to the saucer directly.