The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

         Some few verses are inserted or substituted in the room
         of others.                               --Congreve.
   [1913 Webster]

WordNet (r) 3.0 (2006):

substituting
    n 1: working as a substitute for someone who is ill or on leave
         of absence [syn: subbing, substituting]