That which carries a thing through a sequence of changes. Named for the doing rather than the thing done to.
An agent noun built on process, from Latin processus / procedere (to go forward); a processor is named for carrying a thing through change, not for the thing changed.
That which carries something through a sequence of changes rather than the thing changed itself. The naming is deliberate: the doing, not the done-to, is what earns the -or.