In digital communication systems, the periodicity of timing signals is often disturbed. While timing jitter has been adopted by the International Telecommunication Union (ITU) as the standardized measurement for such disturbances, phase jitter is often used instead in much of the current relevant literature. In this letter, the fundamental concepts of timing jitter and phase jitter are examined and definitions are presented. A nonlinear relationship between timing jitter and phase jitter is developed, and a general condition under which one can be approximated by the other is obtained. This condition is tested against the timing jitter and wander tolerance for digital equipment operating at 2048 kb/s, as specified in ITU-T Recommendation G.823.