Event (usage event)
A single record of product usage — an API call, a token, a compute-second — sent to the billing system and aggregated into totals.
A usage event is a single, discrete record of product usage — one API call, one token processed, one compute-second consumed, one seat activated — sent to the billing system the moment it happens and aggregated into running totals.
It matters because it's the atomic unit everything else in usage-based billing is built from: pricing, entitlements, invoicing and forecasting are all just different computations layered on top of the same stream of events, so the accuracy of the event itself sets the ceiling on how trustworthy every number derived from it can be.
In practice, a usage event carries an idempotency key (so a retried delivery isn't double-counted), a timestamp (so it lands in the correct billing period even if it arrives late), and enough context — customer, event type, quantity — for the pricing engine to know how it should be priced. It's typically sent as a lightweight, fire-and-forget API call that doesn't sit in the product's critical request path.
The common pitfall is treating event emission as an afterthought bolted onto existing code, rather than a deliberate part of the request flow — a usage event that's easy to accidentally skip, duplicate, or lose, because it's sent from a background job with no retry logic, say, quietly undermines every number downstream, and the failure is invisible until a customer or an auditor notices the totals don't add up.