Black Knight

A black knight: like that one time Voldemort tried to take over the world.

In non-Harry-Potter terms, it means an unwelcome takeover, usually of a public company.

For instance, this could happen over a proxy fight (which is when a group of shareholders basically forms a coup to win a corporate vote) or when a company purchases a lot of shares via the stock market for control.

If you were wondering, yes, there is a white knight (think: Dumbledore, Gandalf) and a grey knight (an in-between).