Human Drift is the directional, non-random mutation of meaning-bearing intent as it navigates automated constraint and Context Collapse. Unlike stochastic noise or model drift, which describe degradation or instability in statistical relationships, Human Drift is a structural adaptation: an emergent reorientation of human meaning that preserves intentional continuity while accumulating memory and directionality within signal-only environments. This paper provides the canonical definition, formal properties, dynamic mechanics, and observational framework for Human Drift as the core dynamic process of the Intent–Drift– Meaning (IDM) Spiral Model (Singer, 2026; doi: 10.5281/zenodo.18459128). It distinguishes Human Drift from adjacent phenomena, including concept drift, semantic drift, and behavioral noise, and establishes its role as a foundational mechanism of human– machine mediation. The framework is presented as a diagnostic and analytical construct rather than a prescriptive or normative model.