Agent (noun)

1. A person or organization authorized to act on behalf of another person or organization.
2. A person who acts or does things.
3. A substance or organism that produces a certain effect or result.

Origin:

The word 'agent' comes from the latin word 'agens' which means 'doer' or 'performer'.

Examples:

1. She was hired as a sales agent for the company.
2. The secret agent was on a mission to gather intelligence.
3. The real estate agent showed them several properties.
4. He acted as an agent on behalf of the government.
5. The cleaning agent was used to remove the stain.