Astronomically long time

AEON

Think of an immeasurable span often used in geology or cosmology.

The clue "Astronomically long time" hints at a word meaning an extremely long period, often used in scientific or cosmic contexts. "Aeon" fits perfectly as it refers to an indefinite and vast span of time, commonly associated with geological or astronomical scales. There's no wordplay here—it's a straightforward definition pointing to a term that conveys immense duration.