What type of Waves are "Brainwaves"?

In other words,. What type of waves are Brainwaves categorized in...like are they Traverse waves? Longitudinal waves? What kind are they. I know they are measured in 4 different types, alpha, beta, ect. That is not what I am asking. Lol, do you get what I mean? Does anyone know? I heard they are PGO waves. Im not sure.