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.