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.
Version 1.0 was released in July 2004.
Also the common name for "infectious mononucleosis" AKA "kissing disease."
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