Dilution was the addition of a chemical compound to another, in order to decrease the second compound's potency.
In 2268, Doctor Leonard McCoy prescribed a thergen derivative diluted with alcohol to counter the effects of interphase. Dilution rendered the previously fatal derivative medicinal. (TOS: "The Tholian Web")
In 2370, Data believed the antidote for the Barkonians' radiation poisoning would still be effective, despite being diluted by well water. (TNG: "Thine Own Self")
In 2377, Chellick declared Gar's last supply of cytogenics to have been useless, as the shipment had contained expired lots and diluted samples. (VOY: "Critical Care")