n. existence within time but infinitely into the future; as opposed to eternity, understood as existing outside time
the GNU version of the Collaborative International Dictionary of English
n. Future duration without end; the relation or state of being sempiternal.
The Century Dictionary and Cyclopedia
n. Duration without end; endless duration; perpetuity.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. the property of being perpetual (seemingly ceaseless)
Word Usage
"While in some places at least Plato connected the necessary character of the Forms, including mathematical objects, to eternity, in Aristotle the connection is between necessity and sempiternity."