NumberQ

NumberQ[expr] gives True if expr is a number, and False otherwise.

NumberQ[expr] returns False unless expr is manifestly a number (i.e., has head Complex, Integer, Rational or Real).

NumberQ[Infinity] gives False.

NumberQ[Overflow[ ]] and NumberQ[Underflow[ ]] give True.

You can use NumberQ[x] ^= True to override the normal operation of NumberQ, and effectively define x to be a number.

See The Mathematica Book: Section 2.3.5 and Section 3.1.1.

See also: NumericQ, IntegerQ, MachineNumberQ, TrueQ, Complexes.

Further Examples