The word "threadless" is used to describe the type of fork steerer tube that is used with this form of headset. A "threadless" steerer tube has a smooth surface while a "threaded" steerer tube has threads cut into the top of it so a Threaded headset can be used. Both Threadless and Threaded headsets are typically pressed (friction fit) into a bicycles head-tube, the only major difference is what kind of steerer tube can be used and how the preload is applied to the bearings.
The majority of bicycles use Threadless headsets today while some entry level bikes will use Threaded headsets in order to cut costs.
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