Which of the following is defined as the author's central thought or main point?

The term that best refers to the author's central thought or main point is the main idea. The main idea encapsulates the primary message or argument that an author intends to convey in a text. It serves as the backbone of the piece, summarizing the essential concept that the supporting details and evidence build upon throughout the writing.

While theme relates to the underlying message or insight about life or human nature that can be found in a work, it encompasses broader ideas that may resonate with the reader on a deeper level rather than serving as a straightforward summary of the text's main argument.

Conclusion refers to the final part of a piece, where the author wraps up their ideas rather than presenting the main thought upfront. Summary typically involves a brief recounting of the main points or ideas presented in a text but does not necessarily highlight the specific central thought in the same definitive manner as the main idea does.

Thus, the main idea focuses directly on what the author considers to be the most crucial point, making it the correct choice in this context.