Definition
An event taxonomy is the deliberate design of what events your product emits, what each event means, and what properties accompany them. A good taxonomy is small, descriptive, and stable. A bad taxonomy is ad-hoc, redundant, and changes whenever a new developer adds an event.
Stability matters more than completeness
A taxonomy with 30 well-defined events that stay stable beats a taxonomy with 300 events that drift constantly. Dashboards and funnels are built against names that exist today. Every rename means redoing those dashboards.