since then; ever since; from that time
Refers to a continuous state or repeated action starting from a past event. Often used at the beginning of a sentence or clause.
彼は去年引っ越して、それ以来一度も会っていない。
He moved away last year, and I haven't seen him once since then.
それ以来、毎朝ジョギングを続けている
Ever since then, I've kept up my morning jogging.
Compound of それ (that) and 以来 (since, from). The phrase has been used since at least the Edo period.