Associative

Three elements $x$, $y$ and $z$ of a set $S$ are said to be associative under a binary operation $*$ if they satisfy

$$x*(y*z) = (x*y)*z.$$

Real numbers are associative under addition

$$x+(y+z) = (x+y)+z$$

and multiplication

$$x\cdot(y\cdot z) = (x\cdot y)\cdot z.$$

See also Commutative, Distributive