Cog. A cog is a tooth of a gear or cogwheel or the gear itself.

Cog, COG, CoG, or The Cog  may also refer to:

Science and engineering
 Rear sprocket of a bicycle
 Cog (ship), a type of sailboat from the 10th century onward
 Center of gravity, a spatial point related to an object's center of mass
 Cluster of Orthologous Groups of proteins; see Sequence homology and MicrobesOnline
Conserved oligomeric Golgi complex, that includes COG2, COG4, etc.
INSEE code (also code officiel géographique), a numerical indexing code used by the French National Institute for Statistics and Economic Studies
Mount Washington Cog Railway, the world's first mountain-climbing cog railway
OpenCog, a project that aims to build an open source artificial general intelligence framework
 Chip on Glass; see Liquid-crystal display

Entertainment and media
 C.O.G., a 2013 American drama
 "The Changing of the Guard" (The Twilight Zone), a 1962 episode of the TV series The Twilight Zone
 "Cog" (advertisement), a British television and cinema advertisement launched by Honda
 Star Wars Jedi Quest 8: The Changing of the Guard, the eighth book in the Star Wars Jedi Quest series by Jude Watson
 COG, a 2020 novel by Greg van Eekhout

Fictional characters
 Coalition of Ordered Governments, a fictional organization from the Gears of War series
 Cogs, the fictional antagonists in the MMORPG Toontown Online
 Cogs, the fictional foot soldiers of Power Rangers Machine Empire
 Cog (comics), a character from the Spawn comics

Music
 Cog (band), an Australian progressive rock band
 "Changing of the Guards", a single from Bob Dylan's 1978 album Street-Legal
 The Changing of the Guard (album), 2010 album by indie rock band Starflyer 59
 "Change of the Guard", a song from Steely Dan's 1972 album Can't Buy a Thrill
 "Changing of the Guard", a song from Exodus' 1990 album Impact Is Imminent

Organizations
 Canberra Ornithologists Group, an Australian ornithological organization
 Children's Oncology Group, a National Cancer Institute supported clinical trials group
 Church of God (disambiguation), a name used by numerous, mostly unrelated Christian denominational bodies
 Council of governments, regional bodies that exist throughout the United States
 Covenant of the Goddess, a cross-traditional Wiccan group
 The ISO 3166-1 alpha-3 country code for Republic of the Congo

Other uses
Cog (software), an open source audio player for macOS
Cog (project), a project at the Humanoid Robotics Group of the Massachusetts Institute of Technology
 Continuity of government, defined procedures that allow a government to continue its essential operations in case of catastrophe
 Changing of the Guard (or Guard Mounting), a formal ceremony in which sentries are relieved by their replacements
 Course over ground, the actual path followed by a vessel from A to B as determined by its course

See also
 Change of gauge (disambiguation), with various senses in the airline and rail industries
 C0G, an EIA Class 1 dielectric material with the lowest capacitance/temperature dependence
 CoG (disambiguation)
 COGS (disambiguation)
 Coq, in computer science, Coq is an interactive theorem prover
 Kog (disambiguation)
 System Settings, an application included with macOS
 Settings (Windows), a component of Microsoft Windows