The central theme of a story, essay, or other piece of writing is its key message, topic, or idea. However, there are a number of synonyms that can be used to convey this same concept. For example, a writer might refer to the "primary motif" of a work, the "core subject" or "dominant theme". Other phrases that could be used include "main thread", "principal idea" or "key element". Regardless of the specific wording, the central theme is the underlying concept that ties everything in a piece of writing together, and makes it a cohesive whole.