interaction

Definition: A single biological relationship between two or more entities. An interaction cannot be defined without the entities it relates. Comment: Since it is a highly abstract class in the ontology, instances of the interaction class should never be created. Instead, more specific classes should be used. Currently this class only has subclasses that define physical interactions; later levels of BioPAX may define other types of interactions, such as genetic (e.g. synthetic lethal). Naming rationale: A number of names were considered for this concept, including "process", "synthesis" and "relationship"; Interaction was chosen as it is understood by biologists in a biological context and is compatible with PSI-MI. Examples: protein-protein interaction, biochemical reaction, enzyme catalysis

Source:http://www.biopax.org/release/biopax-level2.owl#interaction