The Collaborative International Dictionary of English v.0.48:

commutativity \com`mu*ta*tiv"i*ty\, n. (Math.)
   the property of a binary operation or operator such that the
   result of one term operating on a second is equal to the
   result of the second term operating on the first.
   [GG]

   Note: If the symbol "*" is taken to be a commutative binary
         operator, then the following equation holds: a*b = b*a
         A common example is the addition operator for real
         numbers, "+": a+b = b+a
         [GG]