Which term refers to the act of damaging the purity or effectiveness of something?

The term that refers to the act of damaging the purity or effectiveness of something is "vitiate." In a broad context, to vitiate means to spoil, impair, or weaken the quality or efficiency of something. This word is often used in legal or philosophical discussions, where the integrity of concepts, arguments, or systems is paramount. For instance, if certain evidence is found to be flawed, it can vitiate the whole argument or case. Vitiation indicates a reduction in value or potency, making it the precise choice for this definition.

The other terms do not convey this meaning. "Wend" means to proceed on one's way or journey, and does not relate to damage or purity. "Wheedle" refers to using flattery or coaxing to persuade someone to do something, rather than impacting purity or effectiveness. "Tumid" describes something that is swollen or inflated, which also does not directly correlate with the act of damaging or impairing. Thus, "vitiate" is the most appropriate term for the act of damaging purity or effectiveness.