Aromatic Compounds:Organic compounds containing one or more planar, cyclic structures with delocalized $\pi$-electrons, exhibiting enhanced stability.
Nucleophilic Aromatic Substitution (S$_N$Ar): A reaction mechanism where a nucleophile replaces a leaving group (typically a halide) on an aromatic ring in the presence of an activating group.
Electrophilic Aromatic Substitution (S$_E$Ar): A reaction mechanism where an electrophile is introduced onto an aromatic ring, typically replacing a hydrogen atom.