1. To convert, to clot, or to curdle a fluid or a substance that is in a solution into a solid or a gel: When Nancy accitdentally poured old and spoiled cream into her coffee, it coagulated by curdling into tiny floating lumps and the coffee tasted awful!
2. Etymology: from Middle French coaguler, from Latin coagulatus, past participle of coagulare, "to cause to curdle"; from cogere, "to curdle, to collect"; from com-, "together" + agere "to drive, to move".