An undefined expression is a mathematical expression that has no answer or cannot be determined. In algebra, an expression is undefined when the denominator of the expression is equal to zero. Any expression with a zero in the denominator or that includes infinity in any part of its expression is considered undefined. Rational expressions are also undefined for values that cause division by zero. In calculus, 0/0 is called an indeterminate form, which has the possibility of assuming any value or no value). In trigonometry, square roots of any negative number are undefined because you can’t multiply two of the same positive nor negative number to get a negative number). In summary, an expression is undefined when:

• The denominator of the expression is equal to zero.
• The expression includes infinity in any part of its expression.
• The expression causes division by zero.
• The expression is a square root of a negative number in trigonometry).