As far as I know, "phase margin" is a control engineering term, and implies the phase difference between input and output when the gain is unity. I don't understand the meaning of this term in opamp terminology. Is there anybody to tell me what that means in op amps?
Phase margin is a general term that applies to any linear electronic system that will operate in a closed-loop fashion.
The open-loop phase margin in an op amp refers to the output phase versus input when the open-loop gain drops to 1.