Bipolar means (as I understand it) an oscillation from depression to mania. I think the transitions typically take days. It seems to me that some mental/emotional system is trying to achieve balance but is not succeeding, and is trapped into keeping going from one extreme to another and back.

That seems to me to be different from "borderline personality disorder" where one is apt to go from one emotional state to another very rapidly (seconds or minutes). The instability is in a way more extreme.