A thing of which there is only ever one instance. Singleness enforced by design rather than by chance.
From single (Latin singulus, one at a time) with the suffix -ton, probably influenced by simpleton; a nineteenth-century card-game coinage later borrowed by computing for a thing of which only one instance may exist.
A thing of which only one instance is ever allowed to exist — singleness held by design, not left to chance.