Hi affrei! This is what I found: (www.answers.com)
Merger The combining of two or more companies, generally by offering the stockholders of one company securities in the acquiring company in exchange for the surrender of their stock.
In spanish: Fusión.
Acquisition
Purchase of an asset, such as real estate, chattels, or securities, with title and rights of ownership passing to the new owner.
In spanish: adquisición.