1. From, away from, off.
2. Prefix applied to electrical units in the CGS-electromagnetic system to distinguish them from units in the CGS-electrostatic system (prefix stat-) and those in the metric system or SI (no prefix).
The definition information for ab- is provided by Stedman's. You can search our medical dictionary here.
Stedman's, part of Lippincott Williams & Wilkins, provides a comprehensive line of health-science publications for healthcare professionals and medical students.