Which word is defined as "calm" or "peaceful"?

The word "placid" is defined as calm or peaceful, which directly aligns with the definition provided in the question. It suggests a sense of tranquility and serenity, often used to describe calm waters or a peaceful demeanor. This term embodies the idea of being undisturbed and free from agitation, making it the best fit for the description.

Other options refer to conditions that are opposite of calm or peaceful. "Stormy" and "turbulent" indicate unease and disturbance, while "chaotic" implies disorder and confusion. Thus, "placid" stands out clearly as the word that embodies calmness and peace, which is precisely what the question asks for.