CoSy can refer to:

• CoSy, an early computer conferencing system developed by the University of Guelph
• CoSy, a compiler development system developed by ACE Associated Compiler Experts
• CoSy, the Cognitive Systems large-scale robotics project.
• CoSy, open array programming language and website, CoSy.com

Cosy can refer to:

• Tea cosy, cloth or wool teapot cover rack
• Mystery cosy, a subgenre of detective fiction often misspelled 'cozy', where violence is downplayed
• Cosy catastrophe, post-apocalyptic science fiction style
• NMR correlation spectroscopy (COSY)

Two-dimensional nuclear magnetic resonance spectroscopy (2D NMR) is a set of nuclear magnetic resonance spectroscopy (NMR) methods which give data plotted in a space defined by two frequency axes rather than one. Types of 2D NMR include correlation spectroscopy (COSY), J-spectroscopy, exchange spectroscopy (EXSY), and nuclear Overhauser effect spectroscopy (NOESY). Two-dimensional NMR spectra provide more information about a molecule than one-dimensional NMR spectra and are especially useful in determining the structure of a molecule, particularly for molecules that are too complicated to work with using one-dimensional NMR.