It says here that trigger context variables are contained in the System.Trigger class.
However, such a class is unknown to the Apex language reference, and
Trigger doesn't seem to behave like a class either. For example, the expected property
Trigger.class is inaccessible from Apex code.
Trigger context contents and usage is reasonably clear, but what exactly is
Background: The idea of using a test stub in place of a real DML-fired trigger context came up while writing tests for a trigger handler class. I would first need to create a stub trigger context, and then execute my trigger in it. I'm yet to find a way to do that.