What is the meaning of sub?

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.
  • synonyms: bomber, grinder, hero, hero sandwich, hoagie, hoagy, Cuban sandwich, Italian sandwich, poor boy, submarine, submarine sandwich, torpedo, wedge, zep
• A submersible warship usually armed with torpedoes.
  • synonyms: submarine, pigboat, U-boat

verb

• Be a substitute.
  • usage: "The young teacher had to substitute for the sick colleague"; "The skim milk substitutes for cream-we are on a strict diet"
  • synonyms: substitute, stand in, fill in