Single Agent:
A single-agent environment where an environment is explored by a single agent. All actions are performed by a single agent in the environment. When there is only one agent in a defined environment, it is named the Single-Agent System (SAS). This agent acts and interacts only with its environment.
Single-agent systems are centralized.
Example: Playing tennis against the ball is a single-agent environment where there is only one player.
Multiple Agent:
If two or more agents are taking actions in the environment, it is known as a multi-agent environment.
A "single-agent system'' should be thought of as a complex, centralized system in a domain that also allows for a multi-agent approach.
Example: Playing a soccer match is a multi-agent environment.