From Longman Dictionary of Contemporary Englishwhite knightˌwhite ˈknight noun [countable] BBBa person or company that puts money into a business in order to save it from being controlled by another company
Examples from the Corpus
white knight• Hartwell had handed his birthright on a silver salver to the rescuing white knight.• Prudential Corporation slipped 3.5p to 195.5p on worries that it could emerge as Pearl's white knight.• If anyone has been set up to be the white knight in the Democratic Party fund-raising debacle, it is Sen.From Longman Business Dictionarywhite knightˌwhite ˈknight noun [countable]FINANCE someone who buys shares in a company to prevent another company taking it over completelyThe shares rose 6p to 648p, indicating that the market continues to expect the intervention of a white knight, possibly from continental Europe. → compare black knight 