Rapid Elasticity is an advantage of Cloud Computing that CompTIA defines as follows: "Rapid elasticity of resources allows you to scale up and down when needed, eliminating the need to maintain additional infrastructure, which may only be utilized under peak workloads." In other words - elasticity gives you the ability to define scaling policies that reduce infrastructure when needed (and save money!). When more infrastructure is needed you can scale up, ensuring fast response times and avoiding outages.
In cloud computing, elasticity is defined as "the degree to which a system is able to adapt to workload changes by provisioning and de-provisioning resources in an autonomic manner, such that at each point in time the available resources match the current demand as closely as possible". Elasticity is a defining characteristic that differentiates cloud computing from previously proposed computing paradigms, such as grid computing. The dynamic adaptation of capacity, e.g., by altering the use of computing resources, to meet a varying workload is called "elastic computing".