I know this is off topic, but it is really interesting.
The world "Termina" is derived from the latin word "termina", meaning to end, finish. Hence, why the area of gameplay is called "Termina" Field, for it would end in only a few days, specifically 3 days before The Moon crashes into the world, bringing destruciton to the area and the areas surrounding it.
Also
terminus: boundary, limit. I believe the word
termina is a derived form, but might [can] also be the feminine form of the word
terminus. Connecting the name solely to mean "the end", as in of the world and disregarding its other meanings might be pushing it a bit; though, it is understandable why a person would do so. Termina Field seems similar to an airport or bus terminal, with pathways leading in the cardinal directions to certain destinations. To the point, I like to believe the portal to Termina could be at any point between the "rabbit hole" or the winding pathway; both appear to have space warping attributes. I place my bets on the rabbit hole. Also perhaps the rabbit hole leads to another sort of terminal, which is a kind of "room" with doors/pathways leading to other
dimensions, Termina being one of them.
Pendio put it nicely that is a bit more abstract than just 'going there.'
To the OP: The events of
Majora's Mask are included on the timeline, because despite having occured in another dimension, they are still events nonetheless. More to the point, those events occured in the life time of the Hero of Time, so it is important they are included. Also, it is hinted that the Hero of Time returned to Hyrule after his exploits in Termina wrapped up.
