Source could mean:

Research 
Source text, in research (especially in the humanities), a source of information referred to by citation
Primary source, firsthand written evidence of history made at the time of the event by someone who was present
Secondary source, written accounts of history based upon the evidence from primary sources
Tertiary source, compilations based upon primary and secondary source.

Organizations 
Source (UNSW), a community service provider that used to operate at the University of New South Wales in Sydney, Australia
The Source by Circuit City, a Canadian electronics store

Technology 
Source, in hydrology, is the origin of water for a river or stream 
Source code for software, see also free software or open-source software
The Source (service), an online service provider founded in 1978 
A terminal in a field effect transistor
Light source, an object emitting light
Sound source, an object emitting sound
Energy sources, are substances or processes with high concentrations of energy
Voltage source, any device or system that produces an electromotive force between its terminals
Current source, an electrical or electronic device that delivers or absorbs electric current
Source engine, a proprietary game engine developed by Valve Software for Half-Life 2
Counter-Strike: Source, a game utilizing the Source engine
Day of Defeat: Source, a game utilizing the Source engine
A SCSI endpoint
Communication source

Music 
The Source, a series of volumes by Graham Kendrick that are compilations of Christian church music
The Source (album), an album by Grandmaster Flash & the Furious Five
The Source, an album by Ali Farka Touré
The Source (musician), the production identity of UK electronic music record producer John Truelove
"The Source", a song by Built to Spill from their 1994 album There's Nothing Wrong with Love
Source, an album released by the band The Duskfall in 2003.

Film and television 
The Source (documentary), a 1999 documentary movie about the Beat generation
The Source (film), a 2002 science fiction movie, also known as The Secret Craft in the UK and The Surge for its American DVD release
The Source, the overarching entity in the machine city of the Matrix universe
The Source (Charmed), a fictional evil being in the television series Charmed
The Source, a character in the short-lived 1978 US television sitcom Quark
Highlander: The Source, the fifth film in the Highlander franchise

Print 
The Source (novel), a 1967 novel by James A. Michener
The Source (magazine), a magazine of hip-hop music and culture
Source (comics), a sentient entity who provides advice to the New Gods in the DC Multiverse
The Source, a book by Art Bell about paranormal matters

Media 
Journalism sourcing, a person, publication or other record or document that gives information
The Source (network), a radio network operated in the 1970s and 1980s by NBC
WVUR-FM, "The Source", Valparaiso University's student-run radio station

Other 
Source theory, any process that generates successive messages can be considered a source of information
Sources of law, the materials and processes out of which law is developed
Sources of international law, the materials and processes out of which the rules and principles regulating the international community are developed
Source (river or stream), the original point from which the river flows
Believers of the Source, a disbanded Faction of the Planescape universe