Byte size answers com gives good definition of byte or So real definition of byte is that it is a basic unit of digital computer On PC this basic unit is 8 nowadays but it used to be 7 Byte is unit in which files or other data is measured on that specific computer of course you can simulate different data types anywhere but on PC most hardware data except flags which are still usually contained in 8 n bit tuples is kept in memory area which is n 8 bits where n is integer In cases when data is kept in smaller units it is called compressed and one piece of compressed units in assembly is usually still contained in 8 n memory area at least when manipulated with assembler s base commands So saying that 1 KB must be 1000 bytes is correct but measuring hardware sizes in bytes is correct only as long as you are using computer which uses 8 bit characters When utf 8 becomes more widespread it s possible that 32 bit bytes will be used instead and this makes KB 4 times larger So KB is system specific notation Saying that byte is 8 bits would be as good as saying that word is 32 bits or worse that integers are 32 bits words have been both 16 and 32 bits depending if archidecture is 16 or 32 bits on 64 bits archidecture it might happen that many languages start to refer 64 bit integers as words So word simply means some datatype which is native enough to do calculations on computer byte mostly means smallest size of non empty file or system managed memory area