The initial side of an angle drawn in angle standard position is the ray lying on the positive x-axis.
In general, viewing an angle as the amount of rotation about the common vertex needed to bring one of two rays into correspondence with the other, the ray being rotated about the vertex is thought of as the initial side.
Portions of this entry contributed by Christopher Stover
More things to try:
Weisstein, Eric W., with contributions by Christopher Stover. "Initial Side." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/InitialSide.html
drawn in angle standard
position is the ray lying on the positive x-axis.
