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Merging is a generic term for combining two or more related sets of data. It is commonly associated with revision control systems when reconciling multiple changes made to a revision-controlled collection of files. Merging multiple data sets is another use of this tag.

Most often, it is necessary when a file is modified by two people on two different computers at the same time. When two branches are merged, the result is a single collection of files that contains both sets of changes.

In some cases, the merge can be performed automatically, because the changes do not conflict. In other cases, a person must decide exactly what the resulting files should contain. Many revision control software tools include merge capabilities.

Merge can be used as a verb ("to merge branches"), but can also be a noun ("this merge will be difficult").

Merging multiple data sets is another use of this tag.

Merge can also refer to the Ruby Hash#merge method for merge hashes or git merge command for git revision control system.

Merge is different from join in datasets which is primarily SQL based. Difference between merge and join for SAS is here and for R is here How to join (merge) data frames (inner, outer, left, right)?

See Wikipedia: Merge Version Control.