Interaction is a kind of action that occurs as two or more objects have an effect upon one another. The idea of a two-way effect is essential in the concept of interaction, as opposed to a one-way causal effect. A closely related term is interconnectivity, which deals with the interactions of interactions within systems: combinations of many simple interactions can lead to surprising emergent phenomena. Interaction has different tailored meanings in various sciences. All systems are related and interdependent. Every action has a consequence.

Casual examples of interaction outside of science include:

• communication of any sort, for example two or more people talking to each other, or communication among groups, organisations, nations or states: trade, migration, foreign relations, transportation,
• the feedback during the operation of machines such as a computer or tool, for example the interaction between a driver and the position of his or her car on the road: by steering the driver influences this position, by observation this information returns to the driver.

Chemistry and medicine

In medicine, most medications can be safely used with other medicines, but particular combinations of medicines need to be monitored for interactions, often by the pharmacist. In molecular biology, the knowledge on gene/protein interaction among themselves and with their metabolites is referred to as molecular pathways.

Interactions between medications (drug interactions) fall generally into one of two main categories:

1. pharmacodynamic : Involving the actions of the two interacting drugs.
2. pharmacokinetic : Involving the absorption, distribution, metabolism, and excretion of one or both of the interacting drugs upon the other.

In terms of efficacy, there can be three types of interactions between medications: additive, synergistic, and antagonistic. Additive interaction means the effect of two chemicals is equal to the sum of the effect of the two chemicals taken separately. This is usually due to the two chemicals acting on the body in the same way. Examples would be Aspirin and Motrin, Alcohol and Depressant, Tranquilizer and Painkiller. Synergistic interaction means that the effect of two chemicals taken together is greater than the sum of their separate effect at the same doses. An example is Pesticide and Fertilizer, the biological effect is devastating. Antagonistic interaction means that the effect of two chemicals is actually less than the sum of the effect of the two drugs taken independently of each other. This is because the second chemical increases the excretion of the first, or even directly blocks its toxic actions. Antagonism forms the basis for antidotes of poisonings. An example is Asparagus and birth control pills.