Error budget
An error budget is the allowable reliability gap between the SLA (customer contract) and the SLO (operational target). If your SLO is 99.9% and you're meeting 99.95%, you have a 0.05% error budget to spend on risky changes, new features, infrastructure migrations, schema rewrites. Error budgets convert reliability from a yes/no debate into a tradeable resource.
The team's relationship with the error budget shapes release cadence: when there's budget, ship boldly; when budget is depleted, slow down and prioritise reliability work. The Google SRE handbook treats burn-down of the error budget as a primary on-call signal, burning faster than expected triggers a freeze on non-reliability work. The tradeable-resource framing is what makes the concept stick organisationally; without it, reliability vs. velocity becomes a recurring philosophical debate.