Understanding is the ability to know and comprehend the meaning, process or reason for a concept, law or person. (e.g. "She had an understanding of God".) It may also mean to have a basic knowledge of a subject, so one can "have an understanding of economics".
An understanding may also refer to an informal agreement about a state of affairs between two or more parties, in diplomacy this may formalized with a "memorandum of understanding".
Compare Specious reasoning.