The mass of a body is a measure of its energy content.

— Albert Einstein, Does the Inertia of a Body Depend upon Its Energy Content? (1905)

Source: http://books.google.com/books?id=lYvz0_8aGsMC&lpg=PP1&pg=PA177#v=onepage&q&f=false

From his 1905 Annalen der Physik paper, as quoted in Max Jammer's 1961 book.

Topics: physics