Which term describes a fictional narrative that typically has a length of under 20,000 words?

The term that describes a fictional narrative typically under 20,000 words is "novella." A novella is a written work that is longer than a short story but shorter than a full-length novel, often ranging from 20,000 to 40,000 words.

While a short story is indeed a brief fictional narrative, it usually has an even shorter word count, often under 7,500 words. Therefore, the short story option would not accurately fit the length criteria laid out in the question.

The general term "narrative" refers to any storytelling format, encompassing various forms such as novels, novellas, short stories, and even poems. It does not specify length or structure, making it too broad for this specific question.

Lastly, a poem is a different literary form focused on rhythmic and often aesthetic qualities, which distinguishes it from narrative prose forms. Thus, while it’s a valid form of literature, it doesn't meet the criteria set by the question regarding fictional narratives of a specified length.