What is the name of the process that changes the phase angle of an RF signal to convey information?
Why Modulation means varying some property of an RF carrier to carry information. Vary the amplitude and you get AM, vary the frequency and you get FM, and vary the phase angle and you get phase modulation. Phase modulation is closely related to FM, since shifting phase over time is the same as a momentary frequency change, which is why a phase modulator driven through an audio shaping network can produce an FM signal.
Watch out Phase inversion is a real term but it just means flipping a signal 180 degrees, as an inverting amplifier does, and it carries no information by itself. Phase convolution and phase transformation are not standard radio terms.
Change amplitude = AM, change frequency = FM, change phase = PM. The word is always modulation.