Mono
Mono is an open-source project that allows developers to build cross-platform applications using the C# programming language and allow .NET applications to run on Linux, macOS, and Windows. Essentially, Mono is a platform that provides a .NET-compatible set of tools, including a C# compiler and a Common Language Runtime (CLR).
Mono has played a pivotal role in the cross-platform development ecosystem, especially, of course, for .NET developers who want to target platforms other than Windows. The distinction between .NET Core and Mono has blurred in recent years with Microsoft’s shift towards more open-source endeavors and its focus on .NET Core (later unified under .NET 5 and onwards), which is inherently cross-platform itself.