(noun)
a large sandwich made of a long crusty roll split lengthwise and filled with meats and cheese (and tomato and onion and lettuce and condiments); different names are used in different sections of the United States
(verb)
be a substitute
The young teacher had to substitute for the sick colleague
The skim milk substitutes for cream--we are on a strict diet
(noun)
a submersible warship usually armed with torpedoes