Contamination was the introduction of a foreign substance or concept to a formerly pure or unaltered substance, either intentional or accidental.
In 2151, Doctor Phlox found the water supply of the Novans contaminated by the irradiated debris from the asteroid impact of 2083. This caused the sickness of the Novans such as Nadet's lung cancer. (ENT: "Terra Nova")
A contaminated drink was said to be spiked. In 2154, Trip Tucker compared the residual effects of Orion pheromones on the crew of Enterprise NX-01 to someone having spiked the water supply. (ENT: "Bound")
In 2266, Spock noted that spectro-readings of Psi 2000 showed no contamination or unusualt elements. Montgomery Scott noted that there might still be some present that were not detectable by their tricorders, which Spock agreed to. (TOS: "The Naked Time")