substituent

 
Pronunciation: /səbˈstɪtjʊənt/

noun

Chemistry
  • an atom or group of atoms taking the place of another atom or group or occupying a specified position in a molecule: displacement of alkyl substituents [as modifier]: the presence of substituent hydrogen atoms

Origin:

late 19th century: from Latin substituent- 'standing in place of', from the verb substituere (see substitute)