Pyridinium refers to the cationic form of pyridine. This can either be due to protonation of the ring nitrogen or because of addition of a substituent to the ring nitrogen, typically via alkylation. The lone pair of electrons on the nitrogen atom of pyridine isn't delocalized, and thus pyridine can be protonated easily. Pyridine's often used as an organic base in chemical reactions, thus the pyridinium ion's produced as the counter ion to the leaving group in the reaction. This complex's often… (More on Pyridinium)