One that comes to the rescue; a savior.
A person or company that rescues a targeted firm from a takeover attempt by buying the firm.
A rescuer; specif., a company that prevents the takeover, viewed as unfavorable, of another company by offering better terms for a merger with it.
A company that comes to the defense of another company that is subject to an unwanted merger attempt. A white knight often will purchase the targeted company in order to prevent its merger with the would-be acquirer.