Material Adverse Change (MAC)

a change in conditions that significantly lowers a company’s value. In mergers & acquisitions, companies will include “MAC clauses” in purchase agreements that alter the parties rights in the event of some event that substantially alters the value of the company that is being sold. A material adverse change is often defined and frequently is considered to be an extremely high threshold. Some definitions, though, in purchase agreements often exclude or carve out certain widespread events beyond the control of the parties (such as terrorism, natural disasters).
See also Material Adverse Event.