Mono is "a comprehensive open source development platform based on the .NET framework that allows IT and ISV developers to build Linux and cross-platform applications with unprecedented productivity." Put simpler, it's an implementation of Microsoft's .NET for Unix, based on the standards for C# and the Common Language Infrastructure that Microsoft submitted to the ECMA.

It's spearheaded by MiguelDeIcaza and "sponsored" (mostly written by) Ximian/Novell. Mono is Spanish for monkey. (Miguel likes monkeys.)

Version 1.0 was released in July 2004.

Also the common name for "infectious mononucleosis" AKA "kissing disease."