Two Micron All-sky Survey

The Two Micron All-sky Survey was a near-infrared survey of the entire sky by two dedicated telescopes, at Fred Lawrence Whipple Observatory, Mount Hopkins, Arizona and Cerro Tololo Inter-American Observatory in Chile, with observations carried out between 1997 and 2001. The sky was imaged in three passbands: J (1.25 μm), H (1.65 μm), and K_s (2.17 μm), i.e., the JHK photometric system. Final data was released in 2003. Targets included galaxies obscured by Milky Way light, brown dwarfs, low mass stars, and general cataloging. It produced a astronomical catalog of over three million objects and many 2MASS designators of objects are now in common use.

The name 2MASS XSC (for 2MASS Extended Source Catalog) is a 2MASS-derived catalog of galaxies and other extended sources.

Note there was a much-earlier (1969) effort with a similar name, the Two Micron Sky Survey. It is not mentioned nearly as often, but its name could confuse someone unaware of it.