The Free On-line Dictionary of Computing (30 December 2018):

start bit

    A bit which signals the start of transmission of
   a character on a serial line.  For an RS-423 signal, the
   line is normally at logical zero which there is no data and
   the start bit is a logical one.  The zero-one transition tells
   the receiver when to start sampling the signal to extract the
   data bits.

   [Is this upside-down?]

   (1995-02-02)