Source : Webster's Revised Unabridged Dictionary (1913)

Substitute \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.

Source : WordNet®

substituting
     n : working as a substitute for someone who is ill or on leave
         of absence [syn: {subbing}]