"The simultaneously-whole and perfect possession of interminable life"
aeternitas est interminabilis vitae tota simul et perfecta possessio (Boethius, De Consol. V)
Yes, this is a good definition of eternity, because eternity is known from two sources: first, because what is eternal is interminable--that is, has no beginning nor end (that is, no terminus either way); secondly, because eternity has no succession, being simultaneously whole.