Four bits (half a byte). The terms nibble and byte were coined in the 1960s by IBM when it switched from six-bit binary-coded decimal (BCD) characters to the eight-bit byte structure in the System/360 series. Nibble was a play on chewing words (nibble, byte, bit) because bit was already coined for "binary digit." The "i" in bite was replaced with "y" to differentiate it from the English word and nibble is sometimes spelled "nybble." See
bit,
byte and
System/360.