main stem
/ˌmeɪn ˈstɛm/noun
1The principal stem of a plant.
2The principal stem of a branching anatomical structure, tree diagram, etc.
3The main line of a railway.
4The main course of a river.
5The principal street of a town; specifically. Broadway, New York.
Origin
Late 17th century; earliest use found in Nehemiah Grew (a1641–1712), botanist and physician. From main + stem.