Definition of 'White Knight'
A white knight is an individual or company that acquires a corporation on the verge of being taken over by forces deemed undesirable by company officials (sometimes referred to as a "black knight"). While the target company doesn't remain independent, a white knight is viewed as a preferred option to the hostile company completing their takeover. Unlike a hostile takeover, current management typically remains in place in a white knight scenario, and investors receive better compensation for their shares.
Investopedia explains 'White Knight'
The white knight is the "savior" of a company in the midst of a hostile takeover. Often a white knight is sought out by company officials - sometimes to preserve the company's core business and other times just to negotiate better takeover terms. An example of the former can be seen in the movie "Pretty Woman" when corporate raider/black knight Edward Lewis (played by Richard Gere) has a change of heart and decides to work with the head of a company he'd originally planned on ransacking.
In addition to white knights and black knights, there is a third potential takeover candidate called a gray knight. As one might guess, a gray knight is not as desirable as a white knight, but more desirable than a black knight.