Marketplace lending is a synonym for peer-to-peer lending and includes all forms and platforms of peer-to-peer lending. When peer-to-peer lending involves lending money from private individuals to private individuals (peer-to-peer), the term marketplace lending also includes institutional investors, i.e. not actual 'peers' in the conventional sense.
Many peer-to-peer platforms are no longer pure intermediaries between peers, but also have institutional investors on the investor side. In the case of the large American platforms such as Lending Club or Prosper, large professional investors already account for around 50% of the investment volume.