1 fountain-head, well-spring, origin, provenance, provenience, inception, start, outset, beginning, root(s), commencement, rise:
We set out to find the source of the Blue Nile. The mineral bauxite is the source of aluminium.
2 originator, author, creator, begetter:
Who is credited as the source of this quotation?
3 authority, documentation; informant, Colloq horse's mouth:
To verify the information, go to the source.