Endianness is the ordering of individually addressable sub-units within a longer data word stored in external memory.
The most typical cases are the ordering of bytes within a 16-, 32-, or 64-bit word, where endianness is often simply referred to as byte order.
Big endian (680x0, Sparc) and little endian (80x86, Rx000) is the two common byte orders. In that case ISO 9660 has data types which allow either and consequently are twice as big.
For example, the 32-bit integer (0x11223344) is represented as the byte sequence (0x44, 0x33, 0x22, 0x11, 0x11, 0x22, 0x33, 0x44), which is essentially a binary palindrome.
The big endian addressing model assigns or maps the lowest address to the highest-order (that is, the most significant or leftmost) data byte of a multibyte-scalar data item.
The little endian addressing model assigns or maps the lowest address to the lowest-order (least significant or right-most) data byte of a multibyte-scalar data item.
The usual contrast is between most versus least significant byte first, called big-endian and little-endian respectively.
Mixed forms are also possible.
The ordering of bytes within a 16-bit word may be different from the ordering of 16-bit words within a 32-bit word, for instance and sometimes collectively referred to as mixed-endian or middle-endian.