WordNet (r) 3.0 (2006):

bit field
    n 1: a field containing only binary characters

The Free On-line Dictionary of Computing (30 December 2018):

bit field

    Part of an item of data, storage location or message,
   identified as a certain number of contiguous bits starting
   at a certain bit position within the data.  Bit position zero
   is usually the least significant bit.

   For example, in an ARM machine code instruction the
   four-bit field at bits 28 to 31 (the four most significant
   bits in the 32-bit word) is the "condition code".

   (2007-03-26)