Developer

n.
  1. One who, or that which, develops.
  2. (Photog.) A chemical bath or reagent used in developing photographs.
  3. (Dyeing) A reagent used to produce an ingrain color by its action upon some substance on the fiber.
  4. a corporation or individual who finances or organizes a real estate development{5}.

By the action of the developer, the latent image on a photographic plate or film, not perceptible to the eye after exposure in the camera, is developed and becomes visible.