Definitions of substitute:

• noun:   a person or thing that takes or can take the place of another
• noun:   an athlete who plays only when another member of the team drops out
• noun:   someone who takes the place of another (as when things get dangerous or difficult)
• verb:   put in the place of another; switch seemingly equivalent items

  Example: "Substitute regular milk with fat-free milk"

• verb:   be a substitute

  Example: "The young teacher had to substitute for the sick colleague"

• verb:   act as a substitute
• adjective:   artificial and inferior

  Example: "Substitute coffee"