A Simultaneity

1. The quality or state of being simultaneous; simultaneousness.
2. (music) More than one complete musical texture occurring at the same time. This first appeared in the music of Charles Ives, and is common in the music of Conlon Nancarrow, and others.

Synonyms

• (state of being simultaneous): concurrentness, synchroneity; simultaneousness

ASTROPHYSICS

Absolute Simultaneity

in relativity: if two events appear simultaneous to one observer, they appear simultaneous to all other observers. This is important because telling time is actually noting the simultaneity of two events.

Example, if we say the train left the station at 7:00, we are saying that two events are simultaneous. The first event is the train leaving the station, and the second event is the clock showing 7:00. If those events are simultaneous for one observer, but not for all observers, then the concept of absolute time has no meaning.