Concatenation; e.g. A || B is the concatenation of bit strings A and B.
Sources:
NIST SP 800-106 under ||

  Concatenation
Sources:
NIST SP 800-133 Rev. 2 under ||

  Concatenation operation; for example, a || b means that string b is appended after string a.
Sources:
NIST SP 800-135 Rev. 1 under ||

  For strings X and Y, X || Y is the concatenation of X and Y. For example, 11001 || 010 = 11001010.
Sources:
NIST SP 800-185 under X || Y

  Concatenation of two strings X and Y.
Sources:
NIST SP 800-56B Rev. 2 under X || Y

  Concatenation of two strings X and Y. X andYare either both bitstrings or both byte strings.
Sources:
NIST SP 800-90A Rev. 1 under X || Y

  The concatenation of bit strings A and B.
Sources:
NIST SP 800-108r1 [August 2022 (Includes updates as of 02-02-2024)] under A || B

  As used in this Recommendation, the concatenation X || Y of bit string X followed by bit string Y is the ordered sequence of bits formed by appending Y to X in such a way that the leftmost (i.e., initial) bit of Y follows the rightmost (i.e., final) bit of X.
Sources:
NIST SP 800-56C Rev. 2

  The concatenation of two bit strings X and Y.
Sources:
NIST SP 800-38B under X || Y
NIST SP 800-38C under X || Y
NIST SP 800-38D under X || Y

  The concatenation of bit strings X and Y.
Sources:
NIST SP 800-38F under X || Y