NB: More recent documentation for Dyalog APL is available at https://docs.dyalog.com/20.0/.

Exclamation Mark;Quote Dot !
Monadic Exclamation Mark means

Factorial 

      ! 3 9 ¯0.11
6 362880 1.07683
Dyadic Exclamation Mark means

Binomial 

      2 1 3 ! 3 10 ¯0.11
3 10 ¯0.0429385

Language Elements