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

1. Be a substitute
   "The young teacher had to sub for the sick colleague";
   - substitute, stand in, fill in
    
2. Act or speak as a replacement (for someone or something)
   - substitute, deputize, deputise [Brit], step in
    
3. Put in the place of another; switch seemingly equivalent items
   - substitute, replace, interchange, exchange

Derived forms: subbings

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

Encyclopedia: Subbing