Since then; from that time onward; used to indicate that something has continued from a certain point in time.
大学を卒業してそれ以来、彼とは会っていない。
I graduated from university and haven't seen him since then.
事故があってそれ以来、車に乗るのが怖くなった。
There was an accident, and since then I've been afraid to ride in cars.
彼女が来て以来、家の雰囲気が明るくなった。
Since she came, the atmosphere in the house has become brighter.