Is “Isthmus” a Scrabble word?
Yes
ISTHMUS
Valid in at least one Scrabble dictionary
Valid in these dictionaries
SOWPODS / CSW
Scrabble UK & International
TWL / NWL
Scrabble North America
Large English Dictionary
General English word list
Meaning of Isthmus
/ˈɪs.məs/
noun
1.A narrow strip of land, bordered on both sides by water, and connecting two larger landmasses.
2.Any such narrow part connecting two larger structures.
3.An edge in a graph whose deletion increases the number of connected components of the graph.