Onomatopoeia refers to the creation of a word that phonetically resembles or imitates the sound that it is meant to describe. Onomatopoeia occurs in all languages. However, each language has its particular way of describing the same sounds. In this case, the second example is the one that best shows onomatopoeia because the author describes the sound of the <em>oobleck</em> as "Plop! Plop!"
The word would be extrinsic. Extrinsic means "<span>not part of the essential nature of someone or something; coming or operating from outside."</span>
