Pseudo-states are a UML 2.3 abstraction for various types of transient vertices used in State Machine diagrams. Pseudo-states are used to express complex transition paths. The following types of pseudo-state are available:
OMG UML Specification
The OMG UML specification (UML Superstructure Specification, v2.1.1, p. 536) states:
A pseudostate is an abstraction that encompasses different types of transient vertices in the state machine graph... Pseudostates are typically used to connect multiple transitions into more complex state transitions paths. For example, by combining a transition entering a fork pseudostate with a set of transitions exiting the fork pseudostate, we get a compound transition that leads to a set of orthogonal target states.