Most fermenting vessels require a stopper to hold an airlock in place or seal it up while moving so what is the technical name of that stopper?
The stopper is called a “bung”. The hole is called the bung hole.
A bung, stopper or cork is a truncated cylindrical or conical closure to seal a container, such as a bottle, tube or barrel. Unlike a lid, which encloses a container from the outside without displacing the inner volume, a bung is partially inserted inside the container to act as a seal. - Bung (Wikipedia)
Bung in the bunghole of a wine barrel