1. Encryption system that uses the same key for encryption and decryption. This key must be kept secret.
Source: Election Terminology Glossary - Draft, National Institute of Standards and Technology (NIST), https://pages.nist.gov/ElectionGlossary/
2. A branch of cryptography in which a cryptographic system or algorithms use the same secret key (a shared secret key).
Source: Explore Terms: A Glossary of Common Cybersecurity Terminology, National Initiative for Cybersecurity Careers and Studies (NICCS), https://niccs.us-cert.gov/about-niccs/glossary