C2

C2 is an alphanumeric designation that appears in multiple domains, where it denotes different concepts depending on context. It does not indicate a single universal entity.

In the Common European Framework of Reference for Languages, C2 is the highest proficiency level, indicating mastery of a language. Users at this level understand almost everything they hear or read, summarize information from various sources, and express themselves with precision and nuance in a wide range of situations.

In chemistry, C2 denotes the diatomic molecule consisting of two carbon atoms. It forms under high-temperature conditions such as combustion and in carbon plasmas, and is studied in spectroscopy and astrochemistry. Its bonding behavior is a topic of ongoing research.

In information security, C2 stands for command and control, the infrastructure that adversaries use to direct compromised devices, issue commands, and exfiltrate data. C2 channels may use various protocols and covert methods to evade detection.

In human anatomy, C2 refers to the second cervical vertebra, also known as the axis. It forms a pivotal joint with the atlas (C1) and enables head rotation.

In military aviation, the C-2 Greyhound is a twin-engine, high-wing cargo aircraft used by the U.S. Navy for carrier onboard delivery.