While working with load balancers it is sometimes required that the client connects to the same backend server all the time. This concept has several names (e.g.: sticky sessions, session stickiness, persistent sessions, persistence, etc…).
Wikipedia explains this concept clearly:
One dilemma when operating a load-balanced service, is what to do if the backend servers require some information (“state”) to be stored “persistently” (across multiple requests) on a per-user basis. This can be a problem if a backend server needs access to information generated by a different backend server during a previous request. Performance may suffer if cached information from previous requests is unavailable for re-use.
One solution is to consistently send clients to the same backend server. This is known as “persistence” or “stickiness”. One downside to this technique is lack of automatic failover, in case one or more backend servers should fail or be taken offline for maintenance. Persistent information is lost if it cannot be transmitted to the remaining backend servers. Citation.