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

See also: substitution

Type of: alter, change, exchange, interchange, modify, replace, supercede, supersede, supervene upon, supplant, work

Encyclopedia: Substitute