WordWeb Online

1. Working as a substitute for someone who is ill or on leave of absence
   - subbing

1. Put in the place of another; switch seemingly equivalent items
   "substitute regular milk for fat-free milk";
   - replace, interchange, exchange, sub [informal]
    
2. Be a substitute
   "The skim milk substitutes for cream--we are on a strict diet"; "The young teacher had to substitute for the sick colleague";
   - sub [informal], stand in, fill in
    
3. Act or speak as a replacement (for someone or something)
   - deputize, deputise [Brit], step in, sub [informal]

Type of: alter, change, exchange, interchange, modify, replace, supercede [non-standard], supersede, supervene upon, supplant, work

Encyclopedia: Substitute