Tracking plan
A tracking plan is the single document that defines every event you track, what each one means, when it fires, and exactly what properties it carries. It's the spec that keeps 'Signed Up', 'signup', and 'sign_up' from all existing at once and quietly wrecking your analytics. Naming conventions, the canonical event list, and who owns each one all live here.
For a founder this is unglamorous and non-negotiable. Analytics rots fast without it: a renamed button, a new dev, or a copy-paste creates a duplicate event, and three months later no chart can be trusted. Write the plan before you instrument, name events consistently (object-action, like 'Subscription Started'), and treat any new event as a change to the plan first. It's cheaper to maintain one clear document than to spend a quarter untangling why your funnel numbers stopped making sense.