! — Factorial / Binomial

Function. Ranks: 0 monadic, 0 0 dyadic

!Y: Y!. Pervasive.

!5                ⍝ 120
!5ₓ               ⍝ 120ₓ

X!Y: binomial coefficient, Y choose X. Pervasive.

2!5               ⍝ 10
2ₓ!5ₓ             ⍝ 10ₓ

Extends through Gamma to non-integral and complex arguments. At a pole, a negative integer, the result is an infinity or NaN, as the gamma function gives it.