Someone who provides financial backing for a startup or entrepreneur, usually in exchange for ownership equity in the company. There are lots of websites that connect startups and angel investors such as Angel Investment Network.
This is an important entrepreneurship term to know in the early stages of your business as they often help move startups from the self-funded stage to a point where the business is able to attract venture capital (underline). Angel investors are also known as private investors, seed investors, or angel funders.