adverb

1. from then until now she arrived Tuesday and has been here ever since
2. at some or any time between then and now; subsequently he was ill last week but has since recovered
3. before the present time; before now; ago they are long since gone

Etymology: ME syns, contr. < sithens, adv. gen. of sithen < OE siththan, for earlier *siththon < sith, after, since (for IE base see side) + thon, instrumental form of thæt, that

1. continuously from (the time given) until now out walking since one o'clock
2. during the period between (the time given) and now; subsequently to many achievements since his election

conjunction

1. during the period from the specified time or event until the present I haven't seen him since we were in high school
2. continuously from the time when I've been lonely since he left
3. inasmuch as; because since you are finished, let's go