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.
OMG UML Specification
The OMG UML specification (UML Superstructure Specification, v2.1.1, p. 387) 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.