Aromaticity and Antiaromaticity: How to Define Them

Aromaticity and antiaromaticity are concepts that are often used to explain and predict the physical and chemical properties of cyclic conjugated compounds. They are associated with 4n + 2 and 4n cyclically arranged electrons that are delocalized and mostly localized, respectively. The large number of papers devoted to these concepts, together with two recent conferences on aromaticity (Aromaticity 2018 and 2025, Mexico) that brought together experts from all over the world to discuss aromaticity and antiaromaticity and their applications testify to their importance, but also to the lack of a simple and easily understandable definition. This review highlights the most important manifestations of (anti)aromaticity by considering selected examples from the literature, chosen to provide us with a clearer picture of these two concepts.