merger

The act or process of merging two or more parts into a single unit.

(economics) The legal union of two or more corporations into a single entity, typically assets and liabilities being assumed by the buying party.

(law) An absorption of one or more estate(s) or contract(s) into one other, all being held by the same owner; of several counts of accusation into one judgement, etc.

(phonology) A type of sound change where two or more sounds merge into one.