"substituting" in the noun sense
1. subbing, substituting
working as a substitute for someone who is ill or on leave of absence
"substituting" in the verb sense
1. substitute, replace, interchange, exchange
put in the place of another switch seemingly equivalent items
"the con artist replaced the original with a fake Rembrandt"
"substitute regular milk for fat-free milk"
"synonyms can be interchanged without a changing the context's meaning"
2. substitute, sub, stand in, fill in
be a substitute
"The young teacher had to substitute for the sick colleague"
"The skim milk substitutes for cream
3. substitute, deputize, deputise, step in
act as a substitute
"She stood in for the soprano who suffered from a cold"
Source: WordNet® (An amazing lexical database of English)
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