Illustrated Glossary of Organic Chemistry

Deactivator (deactivating group): In electrophilic aromatic substitution, an aromatic ring substituent that decreases nucleophilicity and arenium ion stability (relative to a hydrogen atom), thereby causing the reaction to be slower than the same reaction on benzene itself.

When X = NO₂ the rate is less than when X = H, so NO₂ is an deactivator.