Plain English Meaning

It is an extra rating you add to a pilot or instructor certificate you already hold so you can fly or teach in airplanes with two or more engines.

Full Definition

An additional aircraft class rating added to an existing pilot or flight instructor certificate that authorizes the holder to act as pilot in command of, or instruct in, airplanes with more than one engine. The add-on is earned by meeting the aeronautical experience requirements and passing the required practical test for the multiengine class; a separate written knowledge test is generally not required when adding a class rating at the same certificate level already held.

Context Anchor

Seen in flight instructor endorsements when an instructor recommends an for the multiengine add-on practical test.