Definitions for "Head tube"
The front tube of the frame through which the steering tube of the fork passes. The head tube is attached to the top tube and down tube and locates the headset.
Short vertical tube at the very front or "head" of the bike. The front fork fits inside the head tube.
The front tube on a frame that houses the headset and fork steerer tube. The Head Tube Angle is one of the main factors that determines steering quickness in a frame.