Which definition best describes a portmanteau?

A portmanteau is defined as a blend of two or more words into a new one, which is precisely why this choice is the best answer. In linguistics, portmanteaus combine the sounds and meanings of the original words. For example, "brunch" combines "breakfast" and "lunch," creating a new term that reflects a hybrid meal. This characteristic of merging elements is what distinguishes portmanteaus from other types of words, emphasizing their unique formation.

Other definitions mentioned do not capture the essence of a portmanteau. A word used exclusively for poetry refers more to stylistic choices rather than the structural combination of words. A long-form narrative with many characters describes a specific literary genre and has no connection to word formation. A word that has undergone a significant change in meaning indicates a shift over time rather than a creative merging of two originals into something new. Each of these choices reflects different aspects of language or literature but does not encapsulate the specific process involved in creating a portmanteau.