Amazon EC2 Reserved Instances and Other AWS Service Reservation Models
AWS Whitepaper

Amazon ElastiCache Reserved Nodes

Amazon ElastiCache reserved nodes give you the option to make a low, one-time payment for each cache node you want to reserve. In turn, you receive a significant discount on the hourly charge for that node. Amazon ElastiCache provides three standard reserved cache node types (No Upfront, Partial Upfront, and All Upfront) that enable you to balance the amount you pay up front with your effective hourly price.

For more information, see Amazon ElastiCache Reserved Nodes.