Here's a breakdown:

* Xylem is a type of vascular tissue in plants that transports water and dissolved minerals upwards from the roots.

* Xylem vessels are long, hollow tubes formed from dead, interconnected xylem cells. These cells lose their cell contents and their end walls break down, creating a continuous channel for water flow.

* Tracheids are another type of xylem cell, but they are narrower and more elongated than vessels. They also have pits in their cell walls, which allow water to move from one tracheid to another.

So, while individual xylem cells can be referred to as "xylem cells", a group of them working together to form a continuous tube for water transport is specifically called a xylem vessel.