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

Network Time Protocol
NTP

   (NTP) A protocol built on top of TCP/IP that assures
   accurate local timekeeping with reference to radio, atomic or
   other clocks located on the Internet.  This protocol is
   capable of synchronizing distributed clocks within
   milliseconds over long time periods.  It is defined in STD
   12, RFC 1119.