From AMS Glossary
1. The phenomenon of the sun's daily appearance on the eastern horizon as a result of the earth's rotation.
Under normal conditions the upper limb of the sun appears to be on the horizon of an observer at sea level. The parallax of the sun is ignored. The observed time may differ from the tabular time because of a variation of the atmospheric refraction from the adopted value (34') and because of a difference in the height of the observer and the actual horizon.
2. Contraction for "time of sunrise" which is defined by the U.S. Naval Observatory.
Reference: United States Naval Observatory and her Majesty's Nautical Almanac Office Annual. The Astronomical Almanac. U.S. Government Printing Office, Washington, D.C. and HMSO, London, England.