The property of a byte string having its bytes positioned in order of decreasing significance. In particular, the leftmost (first) byte is the most significant byte (containing the most significant eight bits of the corresponding bit string) and the rightmost (last) byte is the least significant byte (containing the least significant eight bits of the corresponding bit string). For purposes of this Recommendation, it is assumed that the bits within each byte of a big-endian byte string are aslo positioned in order of decreasing significance (beginning with the most significant bit in the leftmost position and ending the the least significant bit in the rightmost position).
Source(s):
NIST SP 800-56B Rev. 2