If electrophilic aromatic substitution of a monosubstituted benzene is slower than that of benzene under identical conditions, the substituent in the monosubstituted benzene is called a deactivating group.

eg:

Under identical conditions, Reaction 2 is slower than Reaction 1.  Thus, the nitro group is a deactivating group.

All deactivating groups are electron-withdrawing groups.

Common deactivating groups:

see also activating group, ortho, para directing group, meta directing group