Synergy is the concept that a combined effect of two or more entities is greater than the sum of their individual effects. The term synergy comes from the Attic Greek word συνεργία synergia from synergos, συνεργός, meaning "working together". Synergy is similar in concept to emergence.

In essence, synergy describes a situation where the whole is greater than the simple sum of its parts (i.e., a non-linear addition of force, energy, or effect). It is a fundamental principle observed in various fields, including physics, chemistry, biology, and social sciences. A common example is water, a compound whose properties are distinct and more complex than those of its constituent elements, hydrogen and oxygen.

The concept has a long history, with its roots in physiology and theology before being applied to social psychology and organizational behavior. In Christian theology, for instance, synergism is the idea that salvation involves cooperation between divine grace and human freedom.

In the biological sciences, synergy is seen as a key driver of evolution. The Synergism Hypothesis, proposed by Peter Corning, suggests that the cooperative relationships at all levels of living systems, from genes to social groups, are favored by natural selection because they provide functional advantages for survival and reproduction. This principle also applies to toxicology and pharmacology, where the combined effect of multiple substances, such as drugs or toxins, can be significantly greater than their individual effects, a phenomenon known as drug synergy. This can be both beneficial, as in combination drug therapies, or harmful, as in the case of combined exposure to toxic chemicals.

In a business context, synergy suggests that a team can produce a superior result compared to what its members could achieve individually. This is often a goal in corporate mergers and acquisitions, where the combined entity is expected to be more valuable than the two separate companies. However, potential negative effects such as groupthink can arise in highly cohesive groups, leading to flawed decision-making. Furthermore, in the context of business, the word has been increasingly getting a pejorative connotation due to being used as corporate buzzword and the association with "management-speak".

The words synergy and synergetic have been used in the field of physiology since at least the middle of the 19th century:

SYN'ERGY, Synergi'a, Synenergi'a, (F.) Synergie; from συν, 'with', and εργον, 'work'. A correlation or concourse of action between different organs in health; and, according to some, in disease.

In 1896, Henri Mazel applied the term "synergy" to social psychology by writing La synergie sociale, in which he argued that Darwinian theory failed to account of "social synergy" or "social love", a collective evolutionary drive. The highest civilizations were the work not only of the elite but of the masses too; those masses must be led, however, because the crowd, a feminine and unconscious force, cannot distinguish between good and evil.