isochron
noun
iso·chron ˈī-sə-ˌkrän 
variants or isochrone
ˈī-sə-ˌkrōn
: an imaginary line or a line on a chart connecting points at which an event occurs simultaneously or which represents the same time or time difference
Word History
Etymology
International Scientific Vocabulary is- + -chron (from Greek chronos time)
First Known Use
1881, in the meaning defined above