The component which connects the handlebar to the fork.
There are two basic stem designs, the quill stem and the threadless stem.
The quill stem is the traditional design, found on bikes made before the mid-1990s and inexpensive bikes made since then. The bottom of the stem fits into the steerer tube of the fork.
The threadless stem, also known as A-head stem, was introduced in the mid-1990s. It consists of a single short tube which bolts onto the outside of the unthreaded steerer.