私自 (sī zì): private; personal; secretly; without explicit approval

The company stipulates that employees may not use the official seal without authorization; they must obtain their supervisor's approval.

He took the equipment from the lab without permission and was severely criticized by his advisor as a result.

These documents are confidential; you cannot copy them or take them out of the office without authorization.

The mother discovered that her son had used her phone to top up a game with a lot of money without her permission and was very angry.

In ancient times, officials who privately traded with foreign merchants could be punished.