A Decision is an element of an Activity diagram or Interaction Overview diagram that indicates a point of conditional progression: if a condition is true, then processing continues one way; if not, then another.
This can also be used as a Merge node in that multiple alternative flows can be merged (but not synchronized) to form one flow. The following examples show both of these modes of using the decision element.
Used as a decision:
See UML Superstructure Specification, v2.0, figure 238, p. 321.
Used as a merge:
See UML Superstructure Specification, v2.0, figure 266, p. 344.
OMG UML Specification
The OMG UML specification (UML Superstructure Specification, v2.0, p. 319 (Decision symbol)) states:
"A decision node is a control node that chooses between outgoing flows. A decision node has one incoming edge and multiple outgoing activity edges."
The OMG UML specification (UML Superstructure Specification, v2.0, p. 343 (Merge symbol)) also states:
"A merge node is a control node that brings together multiple alternate flows. It is not used to synchronize concurrent flows but to accept one among several alternate flows...A merge node has multiple incoming edges and a single outgoing edge."