Linear interpolation joins a selected keyframe and the next keyframe with a straight line. The slope between keyframes is constant. For example, the curve in has three keyframes with Linear interpolation.

Three keyframes with linear interpolation
 Except where otherwise noted, this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License
 Except where otherwise noted, this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License