Source : Free On-Line Dictionary of Computing

stop bit
     
        In serial communications, where each bit of the message is
        transmitted in sequence, stop bits are extra "1" bits which
        follow the data and any {parity} bit.  They mark the end of a
        unit of transmission (normally a byte or character).
     
        For example, characters on an {EIA-232} {serial line} may have
        one or two stop bits added.  Some {UART}s even allow for 1.5
        stop bits but one is probably the most commonly used.  A
        serial connection may be described as, for example, "8N1"
        which means eight data bits, no {parity} and one stop bit.
     
        (1995-01-24)