Spinning reserve is any back-up energy production capacity which is can be made available to a transmission system with ten minutes' notice and can operate continuously for at least two hours once it is brought online.
Non-spinning reserve is generating capacity which is capable of being brought online within 10 minutes if it is offline, or interrupted within 10 minutes if it is online, and which is capable of either being operated or interrupted for at least two hours.
Spinning is derived from hydroelectric and combustion turbine terminology. Reserve generator turbines can literally be kept spinning without producing any energy as a way to reduce the length of time required to bring them online when needed.
See also:operating reserve, system reserve, required system reserve, offline/online, back-up power, Independent System Operator, regulation, balancing energy supply, turbine