A somewhat literary or formal word for an endless span of time. Often used in set phrases like 万古不易 (eternally unchanging) or 万古長青 (evergreen). As an adverb, it can mean 'forever' or 'for all eternity'.
This truth is eternally unchanging.
His name will shine for all eternity.
From 万 (ten thousand, myriad) and 古 (old, ancient), literally 'ten thousand ages'. The exact historical derivation is uncertain, but the combination conveys an immeasurably long time.