Definition of sign off:
(n) :
term used to describe the
closing of a
radio or
television station's studios and cessation of a broadcasting signal, usually during the overnight hours
"sign off for the small-town radio station was at midnight."
(v) :
to
log off; to stop using a
computer,
radio, etc., especially to stop
talking; (broadcasting) To cease broadcasting a radio or television signal, usually at the end of a broadcasting day