The Free On-line Dictionary of Computing (30 December 2018):
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-232serial line may have
one or two stop bits added. Some UARTs 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.