What does "sempiternity" mean?

Existence within time but infinitely into the future, as opposed to eternity, understood as existence outside time.

What is a synonym for "sempiternity"?

Common synonyms include: constancy, continuance, durability, eternity, incessancy, infinity.

Pronounced //ˌsɛmpɪˈtɜːnəti//.

/ˌsɛmpɪˈtɜːnəti/ noun

noun ·Rare ·Advanced level

Noun

1
Existence within time but infinitely into the future, as opposed to eternity, understood as existence outside time. uncountable
2
the property of being perpetual (seemingly ceaseless) wordnet
3
Eternity; a seemingly everlasting period of time. literary, uncountable

From sempitern(al) + -ity, from Latin sempiternitas.