dharmamudrā [dhamma-lakkhaṇam] seal of doctrine. Impermanence, suffering and non-self are the three basic seals of dharma. In addition to these seals, the Sūtra-s also mention emptiness as a seal. All these things are referred to as seals because any teaching which does not bear these features cannot be said as a teaching of the Buddha.

Buddhānusmṛti - A Glossary of Buddhist Terms

A Comprehensive Manual of Abhidhamma. IX. 356. Dīgha Nikāya. Mahāparinibbāna. Dhammapada. XX. 277-279.