The left-most bit(s) of a bit string.
Sources:
NIST SP 800-38A
under Most Significant Bit(s)
NIST SP 800-38B
under Most Significant Bit(s)
NIST SP 800-38C
under Most Significant Bit(s)
NIST SP 800-38D
under Most Significant Bit(s)
NIST SP 800-38F