[1] Clustering elasticity is the ease of adding or removing nodes from the distributed data store.
Usually, this is a difficult and delicate task to be done by an expert in a relational database system.
Some NoSQL data stores, like Apache Cassandra have an easy solution, and a node can be added/removed with a few changes in the properties and by adding specifying at least one seed.
[1] Relational databases are most often very inelastic, as they have a predefined data model that can only be adapted through redesign.
[1] The drawback is that the programmer has to take into account that the data model may change over time.