Over

is a primitive dyadic operator which takes two function operands and produces a derived function which pre-processes the argument(s) using the monadic right operand, before applying the left operand on/between the result(s).

Explanation
When the resulting function is used monadically, it has the same behaviour as if the Beside or Atop operator had been used: When the resulting function is used dyadically, both arguments are pre-processed:

Lessons

 * APL Cultivation

Documentation

 * Dyalog
 * J Dictionary, NuVoc