The head angle, also called caster or rake, is with bicycles the angle in which a head tube is tilted in the longitudinal direction with respect to a vertical line.
With bicycles, the term “rake” is common, with four-wheel vehicles the term “caster” is common.
But both terms have the same meaning and function.
Positive rake is when the top of a (possibly imaginary) lineis backward inclined, if it leans forward, there is a negative rake.
Along with the fork offset and trail they determine the most important part of the steering behavior.