1) Synergism 2) Synergy 
The process by which the addition of one agent, which by itself has no toxic effect, increases the toxicity of another agent when exposure to both agents occurs simultaneously.
In medicine, the effect of increasing the potency or effectiveness of a drug or other treatment.
Potentiation is the ability of one chemical to enhance the activity of another chemical to an extent greater than the simple summation of the two expected activities.
<physiology> Increase in quantal release at a synapse following repetitive stimulation. Whereas facilitation at synapses lasts a few hundred milliseconds, potentiation may last minutes to hours. ... <pharmacology> The synergistic action of two drugs, being greater than the sum of the effects of each used alone. ... Origin: L. Potentia =...
(po-ten″she-a´shәn) enhancement of one agent by another so that the combined effect is greater than the sum of the effects of each one alone.
A term used to describe the enhanced toxicity attained by combining two or more toxicants giving more killing power than the sum of the individual toxicities. Used primarily in pharmacology, synergism being commonly used in insect toxicology.Potentiation is the ability of one chemical to enhance the activity of another chemical to an extent greater than the simple summation of the two expected activities.
Increase in quantal release at a synapse following repetitive stimulation . Whereas facilitation at synapses lasts a few hundred milliseconds, potentiation may last minutes to hours.
Type: Term Pronunciation: pō-ten′shē-ā′shŭn Definitions: 1. Interaction between two or more drugs or agents resulting in a pharmacologic response greater than the sum of individual responses to each drug or agent.
[n] - (medicine) the synergistic effect of two drugs given simultaneously
The ability of one chemical to increase the effect of another chemical. 
noun (medicine) the synergistic effect of two drugs given simultaneously