What is the meaning of the word "subject"?

Definitions:

a person who owes allegiance to that nation noun
- A nation has a duty to its subjects.
being under the power or sovereignty of another or others adjective
- subject peoples
cause to experience or suffer or make liable or vulnerable to verb
- I subjected them to my award winning poetry.
likely to be affected by something adjective
- the bond is subject to taxation
make accountable for verb
- I did not want to subject myself to the judgments of others.
one of the two main constituents of a sentence noun
- A sentence often begins with the subject.
something (a person or object or scene) selected by an artist or photographer for graphic representation noun
- a moving picture of a train is more dramatic than a still picture of the same subject
the subject matter of a conversation or discussion noun
- I didn't want to discuss that subject.