Sequence of binary bits that comprise the input, output, State, and Round Key. The length of a sequence is the number of bits it contains. Blocks are also interpreted as arrays of bytes.
Source(s):
FIPS 197 under Block

  A binary vector. In this document, the input and output of encryption and decryption operation are 64-bit block. The bits are numbered from left to right. The plaintext and ciphertext are segmented to k-bit blocks, k = 1, 8, 64.
Source(s):
NIST SP 800-20 under Block

  A subset of a bit string. A block has a predetermined length.
Source(s):
NIST SP 800-22 Rev. 1a under Block

  For a given block cipher, a bit string whose length is the block size of the block cipher.
Source(s):
NIST SP 800-38B under Block
NIST SP 800-38D under Block
NIST SP 800-38F

  A bit string whose length is the block size of the block cipher algorithm.
Source(s):
NIST SP 800-38C under Block

  In this Recommendation, a binary string, for example, a plaintext or a ciphertext, is segmented with a given length. Each segment is called a block. Data is processed block by block, from left to right.
Source(s):
NIST SP 800-67 Rev. 2 under Block
NIST SP 800-67 Rev. 1 [Superseded] under Block

  A data structure containing a block header and block data.
Source(s):
NISTIR 8202 under Block