in geometry, simple words such as "point" and "line" are taken as undefined terms; in arithmetic, some arithmetic operations are called "undefined", such as division by zero and zero to the power of zero; in algebra, a function is said to be "undefined" at points not in its domain -- for example, in the real number system, f ( x ) = x {\displaystyle f(x)={\sqrt {x}}} is undefined for negative x {\displaystyle x} , i.e., no such values exist for function f {\displaystyle f} .