A stem is a thin piece of material which usually connects something. It is commonly thought of as: "The axis of a plant, whether above or below ground, which ordinarily grows in an opposite direction to the root or descending axis." 
STEM in education means programs in science, technology, engineering and mathematics; see National Council of Teachers of Mathematics
Bibliography - Further Reading
- Bailey, Liberty Hyde, The Standard Cyclopedia of Horticulture. London: Macmillan, 1915. 3639 pages. Copyright expired - now in the Public domain.