3dRender.com : Glossary : Direct Light

Direct Light - Light that travels in a straight line from a light source to a surface which it illuminates. All 3D renderers simulate direct light; only renderers that calculate radiosity or full global illumination calculate indirect light. (This is not to be confused with the 3D Studio Max "Direct" or "Target Direct" light sources, which is just an abbreviation for Directional Light, also known as Infinite Lights.)

Indirect Light - Light that has bounced off of other surfaces, or has been scattered or reflected or focused by other surfaces, before illuminating an object. If your renderer does not calculate indirect light for you, you need to simulate it by adding extra fill or bounce lights to your scene.

Source: Digital Lighting & Rendering by Jeremy Birn, definition and 3D images adapted from the book by permission.  This page © 2002 by Jeremy Birn.