The submarine sandwich is the only major American sandwich whose name describes its silhouette rather than its filling, its city, or its method. It is called a submarine because it looks like one: a long, sealed, cylindrical roll, tapered at the ends, riding low and full. That is the literal reading, and it is worth holding onto, because of all the regional words for this sandwich, "submarine" is the one that makes no claim about what is inside. It names the shape and stops there, which makes it the closest thing the family has to a neutral, default term.
What that default describes is the plain cold build. A long roll split lengthwise, layered the full distance with cold cuts and cheese, given lettuce and tomato and onion, and dressed with oil and vinegar or a simple condiment. No griddle, no sauce to soak the bread, no regional ordering grammar; the sandwich is assembled cold and eaten cold, and it is engineered to survive that way. The components are laid the length of the roll so the first bite and the last bite are the same sandwich, and the dressing is there to season and lubricate a dry cold interior rather than to flavor it loudly. This is the version a deli case makes by the dozen and the version most Americans picture when the word is used without a city attached to it: a balanced, portable, room-temperature meal whose whole job is to hold its shape from the counter to the hand for as long as an hour.
Naming is exactly where this sandwich gets complicated, because the same cold build answers to a different word in nearly every region, and the hot griddled and saucy versions branch off from the same roll into their own forms. Those names and those builds are a map worth drawing carefully, and they deserve their own articles rather than being crowded in here.