What is the meaning of substituting?

noun

• Working as a substitute for someone who is ill or on leave of absence.
  • synonyms: subbing

verb

• Put in the place of another; switch seemingly equivalent items.
  • usage: "the con artist replaced the original with a fake Rembrandt"; "substitute regular milk with fat-free milk"; "synonyms can be interchanged without a changing the context's meaning"
  • synonyms: substitute, replace, interchange, exchange
• 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, sub, stand in, fill in
• Act as a substitute.
  • usage: "She stood in for the soprano who suffered from a cold"
  • synonyms: substitute, deputize, deputise, step in