An Execution Environment is a node that offers an execution environment for specific types of components that are deployed on it in the form of executable artifacts. This is depicted in a Deployment diagram.
Execution Environments can be nested; for example, a database Execution Environment can be nested in an operating system Execution Environment. Components of the appropriate type are then deployed to specific Execution Environment nodes.
OMG UML Specification
The OMG UML specification (UML Superstructure Specification, v2.1.1, p.210) states:
... an ExecutionEnvironment is ... usually part of a general Node, representing the physical hardware environment on which the ExecutionEnvironment resides. In that environment, the ExecutionEnvironment implements a standard set of services that Components require at execution time (at the modeling level these services are usually implicit). For each component Deployment, aspects of these services may be determined by properties in a DeploymentSpecification for a particular kind of ExecutionEnvironment.