In the k programming language, atom is the monadic case of @.

@x returns 1 if x is an atom, 0 otherwise; atoms are single characters, integers, floats (floating point numbers), symbols, dictionaries, null, and lambdas.

Non-atoms are vectors, arrays, and lists of all types.

Illustrated at the k command prompt (user input indented 2 spaces, results flush left):

  atomlist:(1;-1;0.0;.();_n;;`a;"a")
  atomlist
(1;-1;0.0;.();;;`a;"a")
  @atomlist
0
  @:'atomlist
1 1 1 1 1 1 1 1
  @1 2 3
0
  @"cat"
0
  a:3 2#!3*2
  a
(0 1
 2 3
 4 5)
  @a
0
  @:'a
0 0 0
  @:''a
(1 1
 1 1
 1 1)