The best definition of syntax is:
"The way in which words and phrases are arranged to form sentences."
Syntax refers to the set of rules and principles that govern how words and phrases are combined to create grammatically correct sentences in a language. It encompasses the study of sentence structure, word order, and the relationships between different elements of a sentence. The other options provided - unconventional grammatical structures used in poetry, purposeful omission of punctuation for effect, and the order of paragraphs or stanzas - are related to language and writing, but do not specifically relate to the definition of syntax.