Simple Definition:

A sanction is a penalty [a consequence for breaking a rule or law] or punishment imposed by a government or authority [a person or organization that has power and control] to express disapproval or to enforce compliance with rules or laws.
For example:

• A government can impose economic sanctions (restrictions on trade and financial transactions) on a country.
• A student can receive a sanction for cheating on a test, such as a failing grade or academic probation [a period of time during which a student must meet certain requirements to continue their studies].

Very Simple Definition:

A sanction is a punishment given by a government or authority when someone does something wrong.

For example:

• When a person breaks the law, they can be fined or sent to jail as a sanction.
• If a student cheats on a test, they might get a lower grade or have to retake the test as a sanction.