To clarify (a bit)
Yes, of course yaw angle is in relation to a notional reference frame e.g. in stability theory, tells you how far the fuse axis has rotated (in the x-y plane) from a given (initial) direction. Notation = Capital Phi
Sideslip angle (angle to relative wind, freestream directon) = Beta
Yaw originally came from maritime navigation
Originally Posted by Wiki on Yaw, pitch, Roll
In maritime navigation only the
yaw angle is important. In fact, the word has a
nautical origin, with the meaning of "bending out of the course". Etymologically, it is related with the verb 'to go'
[5]. It is typically assigned the shorthand notation ψ.
[6] It is defined as the angle between a vehicle's heading and a reference heading (normally true or magnetic
North)*.
* But not necessarily, that just happened to be a convenient reference system for navigational purposes
Wilbur Wright is credited with
inventing the yaw string, but there is little doubt it was simply seen to be used on sailing ships and yachts, in fact everywhere for many centuries
