A sector-by-sector, bit-by-bit copy of a physical hard drive or a logical drive.
See Bitstream copy: Bit stream backup (also referred to as mirror image backup) involves the backup of all areas of a computer hard disk drive or another type of storage media. Such a backup exactly replicates all sectors on a given storage device. Thus, all files and ambient data storage areas are copied. Bit stream backups – sometimes also referred to as “evidence grade” backups – differ substantially from traditional computer file backups and network server backups.