The white knight is a chess-piece, often associated with the abstract battle of good against evil. This is the origin of most of the other meanings.

In business, white knight refers to a potential acquirer (firm that wishes to acquire ownership of another firm) of a firm. The intention of the acquisition is to circumvent the hostile takover of the target firm by a third firm, which is perceived to be less favorable.

For example:

Company A (Acquirer) is trying to acquire ownership of Company T (target) in order to stop Company H (less welcome) from acquiring ownership of Company T.

In this scenario, Company A would be acting as the white knight.

The White Knight is a character in Through the Looking Glass by Lewis Carroll. Based on the chess-piece, this character is a bumbling do-gooder with a penchant for useless inventions.