Is there a word for something that gets "acted upon"?

For instance, say I have two individuals and one is active, the other passive. I know I can call the active person the "actor"—he "acts upon" the passive person. But what do I call the "acted upon"? Is there a single word to describe this?

The term is Patient, which is formed from the same semi-deponent Latin verb (patior, patere, passus 'suffer, endure') as Passive. Most grammatical terminology comes from Latin, so if you want to make up a linguistic term, get a Latin dictionary.

In a prototypical English active transitive sentence,

• the grammatical relation Subject has a semantic role Agent (< ago, agere, egi, actus 'do'),
  while
• the grammatical relation Direct Object as the semantic role of Patient.

In Functional Grammar the person or thing acted upon is the Patient.

This may seem an odd use of the word, but it is not, in fact, a coinage of modern linguistics. Patiens (the present participle of pati, “to undergo, endure, suffer”) and agens were literal Latin translations of Aristotle’s terms, and the opposition has been used in English since the 16th century:

The eye of the man is the arrow, the bewtie of the woman the white, which shooteth not but receiueth, being the patient, not the agent. —Lyly, Euphues 1580

Love or hate, applaud or condemn, the agents and the patients of [Shakespeare's] mundane scheme. —Swinburne, Essays & Studies 1875