These are the "summary" objects described in SUM$ in the data dictionary that are used for the query rewrite functionality. They show up as UNDEFINED because their type number (42) is not listed in the decode statement on OBJ$.TYPE# in the DBA_OBJECTS family of views.

I see many objects in USER_OBJECTS having OBJECT_TYPE as "UNDEFINED". Object names correspond to snapshots for the tables getting replicated. Any ideas?