Service Level Agreement (SLA) is a part of a service contract where a service is formally defined.
Particular aspects of the service - scope, quality, responsibilities - are agreed between the service provider and the service user. A common feature of an SLA is a contracted delivery time (of the service or performance).
Secondary usage of the term: We've recently observed that some companies have started using the term SLA internally as a formal accountability agreement between marketing and sales teams. Here's a more in-depth article on this topic.