Pyridinium is the protonated form of pyridine. The lone pair of electrons on the nitrogen atom of pyridine is not delocalized, and thus pyridine can be protonated easily. Pyridine is often used as an organic base in chemical reations, thus the pyridinium ion is produced as the counter ion to the leaving group in the reaction. This complex is often insoluble in the organic solvent, so precipitation of the pyridinium leaving group complex is an indication of the progress of the reaction.
The pyridinium ion also plays a role in Fridel-Crafts Acylations. When pyridine is included, it forms a complex with the electrophillic acylium ion, rendering it even more reactive.