A *(term) whose name is the contraction of "binary digit", is the maximal amount of _(information) of a system that has two possible states (which maximum is reached when the two states are equally probable).

For instance, such system can be a digit in radix two, that can be zero or one, or a truth value in classical (boolean) logic, that can be true or false, or current in some digital electronic circuit, that can either pass or not, or a door that is either well-closed or (at least partially) open, or a switch that is either on or off.

The bit is used in computer science as the basic unit to measure the size of _(memory) and of memory-bound objects in computers.

Other well-known units of information, used to measure memory sizes, are the byte, which is eight (8) bit, the word, which greatly depends on the computer architecture used, though it is most often a multiple of a byte, and was generally found to be 32-bit (4 bytes) in computers of the 1990's, though more recent architectures make it 64-bit (8 bytes).

Memory is then measured in multiples of bits (b), bytes (B), or words (W), as identified by an appropriate prefix, which denotes a power of two: K (kilo-) represents 1024, which is 2^10 and a bit more than one thousand, so that one kilobit is 1024 bit or 128 bytes; one kilobyte (1 KB) is 1024 bytes, or 8192 bits. M (mega-) represents 1048576, which is 1024*1024, or 2^20, and a bit more than one million, so that 1Mb is 1048576 bits, or 128KB; 1MB = 1024 KB = 1048576 bytes = 8 megabit. G (giga-) represents 2^30, or 1024*1024*1024, a bit more than 1 billion. 1 GB = 1024 MB. T (tera-) represents 2^40, or 1024*1024*1024*1024, some more than one trillion. 1 TB = 1024 GB = 1048576 MB. 

The above section should be rewritten, since there are now semi-standard clarifying prefixes: KiB, MiB, GiB, TiB, PiB, EiB, for powers of two, whereas kB, MB, GB, TB, PB, EB, will be unambiguously powers of ten.