Illustrated Glossary of Organic Chemistry

Directing group (director): In electrophilic aromatic substitution, a benzene ring substituent that influences benzene ring nucleophilicity and arenium ion stability, causing the incoming electrophile to favor attack at particular positions on the benzene ring.

Bromination of nitrobenzene via electrophilic aromatic substitution gives mostly the meta product, so the NO2 group is a meta director.