Unlike functions, which take one or two arguments depending on context, each APL operator always takes only one operand, or always takes two.
Although the resulting function can be called with one or two arguments, an operator is called "monadic" or "dyadic" based on how many operands it takes.
⌊/ 2 0 3 5 4 ⍝ Reduction (fold)
0
2 ⌊/ 2 0 3 5 4 ⍝ Windowed reduction
0 0 3 4
-∘÷ 0.5 ⍝ Composition
¯2
3 -∘÷ 0.5 ⍝ Weird skewed composition
1