Contamination was the introduction of a foreign substance or concept to a formerly pure or unaltered substance, either intentional or accidental.
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")
In 2366, the water supply on Beta Agni II was contaminated with tricyanate, necessitating the introduction of hytritium in order to neutralize it. (TNG: "The Most Toys")