What does "substitution" mean?

Additional senses

1. 2.The office or authority of one acting for another; delegated authority. [R.] Shak.
2. 3.(Civil Law) The designation of a person in a will to take a devise or legacy, either on failure of a former devisee or legatee by incapacity or unwillingness to accept, or after him. Burrill.
3. 4.(Theol.) The doctrine that Christ suffered vicariously, being substituted for the sinner, and that his sufferings were expiatory.
4. 5.(Chem.) The act or process of substituting an atom or radical for another atom or radical; metethesis; also, the state of being so substituted. See Metathesis.