'sign off' definitions:

Definition of 'sign off'

From: WordNet
verb
Cease broadcasting; get off the air; as of radio stations

Definition of 'sign off'

From: GCIDE
  • sign off \sign` off"\ (s[imac]n` [add]f"), v. i. To terminate a communication, especially one conducted by radio waves; to terminate a broadcast over radio or television; as, this is your reporter in Cairo, signing off. [PJC]