Isthmus of Tehuantepec
Noun
Meaning
the narrowest part of southern Mexico is an isthmus between the Bay of Campeche on the north coast and the Gulf of Tehuantepec on the south coast
Source: WordNetExamples
During the pre Classic era, it is known that most of Chiapas was not Olmec, but had close relations with them, especially the Olmecs of the Isthmus of Tehuantepec.