The term "by" is a preposition used to indicate the agent performing an action or the means through which something is done. For example, in the sentence "The book was written by J.K. Rowling," it shows that J.K. Rowling is the author of the book. Additionally, "by" can denote proximity or location, as in "The park is by the river." Here, it indicates that the park is situated near the river. This versatile word helps clarify relationships between different elements in a sentence.