In general, the act of uniting separate things.  Specifically -

1.  In corporate law, the absorption of one corporation into another.  The surviving corporation acquires all the assets and liabilities of the corporation getting absorbed.  The joining of non-corporate entities such as associations may sometimes be called a merger as well.

2.  In civil procedure, the principle that a final judgment for the plaintiff brings together all claims involved in the lawsuit.  As a result, the plaintiff can only enforce the judgment awarded - and cannot bring any of the claims again because the award seems too small.  This effect of a final judgment is called merger.

3.  In criminal law, the absorption of a lesser included offense into a more serious offense if a defendant is charged with both.  The purpose of merger in criminal cases is the avoidance of double jeopardy.  See Merger doctrine.

4.  In property law, the absorption of a lesser estate (e.g., lease of land) into a greater estate (e.g., fee simple ownership of the same land) when both estates become property of the same person.

5.  In contract law, the absorption of an inferior form of contract into a superior form of contract on the same subject matter.  Thus, an oral agreement discussing some business deal merges into the final written agreement on the same deal; any terms of the oral agreement usually cannot be enforced if contrary to the terms of the written agreement.  See Integration.

6.  In contract law, the modification of some duty or obligation - often resulting from changes in surrounding circumstances or operative facts.