MonoBehaviour
The base class behind nearly every Unity script — the thing that turns a C# class into a component on a game object.
The name comes from Mono, the open-source .NET runtime Unity built its C# support on; the British spelling comes from the company's Danish roots. You never create one with new. You add it to a GameObject, and the engine then calls Awake, Start, Update, FixedUpdate and LateUpdate in a fixed order on every pass of the game loop.
Those methods aren't virtual. Unity finds them by name when the script loads, so a misspelled Updat compiles cleanly and silently does nothing. The class also owns StartCoroutine, which taught a generation of game programmers to write waiting logic with yield long before async was commonplace. Thousands of objects each with their own Update get expensive — which is roughly half the reason DOTS exists.