key·stone 
(kēstōn′)
(kēstōn′)n.
1. Architecture A wedge-shaped stone at the top of an arch, often larger or more ornamented than the other voussoirs. Also called headstone.
2. The central supporting element of a whole.
- electrostatic unit
- electrosurgery
- electrosurgical
- electrotherapeutics
- electrotherapy
- electrothermal
- electrothermally
- electrotonic
- electrotonus
- electrotype
- electrotyper
- electrotypic
- electrovalence
- electrovalency
- electrovalent
- electrovalent bond
- electroweak
- electrum
- electuary
- eleemosynary
- elegance
- elegancy
- elegant
- elegantly
- elegiac