What is an opposite word of subject?

There isn't one single perfect opposite for "subject" because it has multiple meanings. Here are some possibilities depending on the context:

If you're talking about a topic of study or discussion:

* Object

* Theme

* Focus

If you're talking about a person under discussion or control:

* Object (especially in grammatical context)

* Authority

* Ruler

If you're talking about a person who is being experimented on:

* Experimenter

* Researcher

It's important to consider the context to choose the most appropriate opposite word for "subject."