Merriam-Webster's Collegiate® Dictionary, 11th Edition
SINCE
I. adverb
Etymology: Middle English sins, contraction of sithens, from sithen, from Old English siththan, from sīth tham after that, from sīth after, late + tham, dative of thæt that; akin to Old High German sīd later and perhaps to Latin setius to a lesser degree
Date: 15th century
from a definite past time until now , before the present time ; ago , after a time in the past ; subsequently ,
II. conjunction
Date: 15th century
at a time in the past after or later than , when , in view of the fact that ; because ,
III. preposition
Date: circa 1530
in the period after a specified time in the past ; from a specified time in the past