I always thought that the definition of slat was:
"Lift modifying and adjustable wing element on the leading edge of main airfoil."
and the definition of flap was:
"Lift modifying and adustable wing element on the trailing edge of main airfoil."
However, there are also leading edge flaps, also known as Krueger flaps, and they are located on the leading edge.
What gives these elements the name of "flap"? What is the main difference to slats?