1. A large mass of intrusive igneous rock believed to have solidified deep within the earth: Granite is one good example of batholite, also termed "batholith", which was formed by an intrusion of magma at extreme depth and having been exposed after erosion.
2. A large emplacement of igneous intrusive (also called plutonic) rock that forms from cooled magma deep in the earth's crust: Batholites, or batholiths, are composed of multiple masses, or "plutons", of magma that moved toward the surface from a zone of partial melting at the base of the earth's crust.

While moving, these plutons of relatively buoyant magma are called plutonic diapirs. Diapirs commonly intrude vertically upward along fractures or zones of structural weakness through more dense overlying rocks because of density contrast between a less dense, lower rock mass and overlying denser rocks.

Because the diapirs are liquefied and very hot, they tend to rise through the surrounding country rock, pushing it aside and partially melting it.

Most diapirs do not reach the surface to form volcanoes, but instead slow down, cool and usually solidify five to thirty kilometers underground as plutons. Therefore the use of the word "pluton" is in reference to the Roman god of the underworld, Pluto.