MCLoaders, or "Loaders", are the equivalent of the server software being run in order to allow players to play Minecraft on a remote server.

Examples of what we consider as MCLoaders to be would include Paper, Folia, Fabric, Forge, or Sponge. loader

DANGER

Proxies such as Velocity, Bungeecord, or Waterfall, are never considered as "Loaders" because they don't actually hold any data related to a Minecraft instance.

While reading the RustyConnector docs, the word "Loader" is often used to refer to not only the Minecraft server itself but also the instance of RustyConnector running on it.