sub

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.

A submersible warship usually armed with torpedoes.

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”