A type of cold or hot sandwich made from a cylindrical bread roll split lengthwise and filled with meats, cheeses, vegetables, and condiments.
Called a sub or, in Philadelphia, a hoagie.
The featured photo is of a collection of meat-filled submarine sandwiches sliced horizontally into halves.
Subs can be filled with a wide range of ingredients, including meats (like turkey, ham, roast beef, or chicken), cheeses, vegetables (lettuce, tomatoes, onions, peppers, etc.), and condiments (mayo, mustard, vinaigrette, etc.).
Of course, the most ubiquitous fast-casual chain that sells submarines as food is Subway.