I have been coming across the term collaborator in software literature, and I have been using it too in my own writings, but without having seen it defined. I tried searching for its definition, but could not find any. In UML the term collaboration is vaguely described, but not the term collaborator. After asking on Software Engineering Stack Exchange I was pointed to what is in almost all certainty the original definition, but it turns out that it is very old, and slightly problematic, so I thought I should provide a modern definition here, at the very least for use in my own writings.
Here it goes:
A collaborator is a component invoked by another component to do a job.
(And since the context is software, these are, of course, software components.)