This paper provides the axiomatic and structural foundation of the Emotional Tension Index (ETI). ETI conceptualizes emotion not as a subjective internal state, but as an event-generated field arising from rule-based interactions between institutions and individuals. The framework introduces a formal three-layer structure—ETI-A (axiomatic definition), ETI-G (geometric configuration), and ETI-D (field divergence)—to enable non-narrative, non-self-report measurement of emotional tension. The paper is domain-general and serves as the foundational specification for subsequent applied and computational ETI studies. This Zenodo record constitutes the version of record for the ETI axiomatic framework.