Per Reuter & Reuter, an Assistenzarzt is a resident (physician).

The term "resident" is also supported by the following site (which might be quite helpful - check it out!)
senior resident - Assistenzarzt im dritten Weiterbildungsjahr
second year resident (junior resident) -Assistenzarzt im zweiten Weiterbildungsjahr
first year resident - Arzt im ersten Weiterbildungsjahr, mit dem AiP vergleichbar
homepage.ruhr-uni-bochum.de/Andreas.Roth/us2.html

Some further references:
Phillip Rehbein, bmh, Technical University Hamburg-Harburg - [ Neu! Diese Seite übersetzen ]
... PROFESSIONAL ACTIVITIES: 1996-1997 Internship (AiP) and Resident Physician (Assistenzarzt)
at St. Georg Hospital, Department of Surgery Department of Surgery ...
134.28.202.17/bim/Staff/pr.htm

[PDF] www.unibas.ch/reformmed/publications/AKK_basel/docs/AKK_int...
... role of physician- in-training (resident physician) in either a clinical or a ... für
das eigentliche Studienziel (Assistenzarzt) am besten geeignet 90

And here is a brief outline of the tasks that an Assistenzarzt must carry out:
Assistenzarzt/-ärztin
Aufgabenbereich: · Patientenversorgung · Mitarbeit in der Lehre und klinischen Forschung
Voraussetzung: · Approbation · Promotion · Interesse an Lehre und klinischer Forschung
www.dgi.mwn.de/stellen/aa01.htm

As you can see, this goes well beyond the tasks and training of a physician assistant (for more on physician asssistants, cf. www.ashs.edu/discpl_pa.htm).

HTH