In mathematics, defined and undefined are used to explain whether or not expressions have meaningful, sensible, and unambiguous values. Whether an expression has a meaningful value depends on the context of the expression. For example the value of 4 5 is undefined if a positive integer result is required.Sometimes an expression is not well defined but the degree of undefinedness can be qualified in a useful way. There may be more than one possible value, or extra information might help to determine the value for instance.