A Merge Node brings together a number of alternative flow paths in Activity, Analysis and Interaction Overview diagrams. For example, if a Decision is used after a Fork, the two flows coming out of the Decision must be merged into one before going to a Join; otherwise, the Join waits for both flows, only one of which arrives.

A Merge Node has multiple incoming edges and a single outgoing edge. The edges coming into and out of a Merge Node must be either all object flows or all control flows. 

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OMG UML Specification

The OMG UML specification (UML Superstructure Specification, v2.1.1, pp. 385-386) 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. "