Technically speaking, coconuts are seeds. According to the Library of Congress, they are classified as a one-seeded drupe, which is a single seed encased in a hard outing coating.
In addition to being classified as seed, coconut is also a fruit and a nut. A coconut, however, varies from other foods classified as nuts because of the way in which it pollinates from within rather than decaying from the outside. Coconuts are comprised of three layers, the outermost of which is typically removed before they are sold commercially. Scientists do not know where or when coconuts originated though it is suspected that they may have been Polynesian in origin.