1. [n]  a person or thing that takes or can take the place of another
2. [n]  someone who takes the place of another (as when things get dangerous or difficult); "the star had a stand-in for dangerous scenes"; "we need extra employees for summer fill-ins"
3. [n]  an athlete who plays only when another member of the team drops out
4. [v]  put in the place of another; switch seemingly equivalent items; "the con artist replaced the original with a fake Rembrandt"; "substitute regular milk with fat-free milk"
5. [v]  be a substitute
6. [v]  act as a substitute for

1. \Sub"stit"ute\, n. [L. substitutus, p. p. of
   substituere to put under, put in the place of; sub under +
   statuere to put, place: cf. F. substitut. See {Statute}.]
   One who, or that which, is substituted or put in the place of
   another; one who acts for another; that which stands in lieu
   of something else; specifically (Mil.), a person who enlists
   for military service in the place of a conscript or drafted
   man.
      Hast thou not made me here thy substitute?  --Milton.
      Ladies [in Shakespeare's age] . . . wore masks as the sole
      substitute known to our ancestors for the modern parasol.
                                                  --De Quincey.
   

2. \Sub"stit"ute\, v. t. [imp. & p. p. {Substituted}; p.
   pr. & vb. n. {Substituting}.] [See {Substitute}, n.]
   To put in the place of another person or thing; to exchange.
         Some few verses are inserted or substituted in the room
         of others.                               --Congreve.
   

(SUB) ASCII character 26.