Pi-pi interaction is commonly used to describe the attractive force between two parallel planar aromatic rings. However, there are a few synonyms that can be used to describe this interaction. One of them is pi-stacking, which describes the interaction between stacked aromatic molecules. Another synonym is pi-pi stacking, a term commonly used in organic chemistry. The interaction can also be described as pi-cloud interaction, which emphasizes the interaction between delocalized electrons in the aromatic rings. Additionally, pi-electron interaction is often used to describe the attraction between pi-electron clouds. Overall, these synonyms provide alternate ways to describe the same phenomenon, highlighting the importance of clear communication in scientific fields.