Cloud cost
Reserved instance
A commitment to a specific instance type in a specific region or zone, discounted versus on-demand pricing.
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Definition
A reserved instance is the older, more rigid form of committed use discount: you commit to a specific instance family and size, and in exchange pay a lower hourly (or fully upfront) rate than on-demand for the term of the reservation. Because it's tied to a specific shape, it earns a deeper discount than a savings plan but doesn't flex if your instance mix changes.
RIs are worth revisiting whenever an architecture shifts. A reservation for an instance type you've since migrated away from stops paying for itself and just becomes stranded commitment.
Where it shows up
A reserved instance appears on the bill as its own charge line tied to a specific instance family, size, and region or Availability Zone. AWS Cost Explorer's RI reports break it out from on-demand usage of the same instance type, showing the discounted rate next to the reservation's remaining term.
What makes it expensive
A reservation outlives the architecture that justified it more often than anyone expects. Migrate off that instance family and the reservation keeps billing at the committed rate while covering nothing. Utilization on that one reservation falls, the account total barely moves, and nothing draws attention to it. That is the case for reviewing reservations one at a time rather than in aggregate.
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