Byte addressing refers to hardware architectures which support accessing individual bytes of data rather than only larger units called words, which would be word-addressable.
The basic unit of digital storage is called a bit. In most common computer architectures, 8 bits are grouped together to form a byte. Byte addressable memory refers to architectures where data can be accessed 8 bits at a time, irrespective of the width of the data and address buses.
Many common architectures can address more than 8 bits of data at a time. For example, the Intel 386SX processor can handle 16-bit (two-byte) data, since data is transferred over a 16-bit bus. However, data in memory may be of various lengths. A 64-bit architecture machine might still need to access byte-sized data over its 64-bit address line. Such memory, which is accessible in 8-bit segments, is called Byte-Addressable Memory.
Hence it is called byte-addressable memory.