The part of the paragraph or paper to which all the other ideas relate. See topic sentence and thesis.

a complete thought (a sentence) which expresses the most important idea of a passage

In informational or expository writing, the most important thought or overall position. The main idea or thesis of a piece, written in sentence form, is supported by details and explanation. See Theme, Thesis

The point the author is making about a topic. Topic and main idea are not the same.

The gist of a passage; central thought; the chief topic of a passage expressed or implied in a word or phrase; the topic sentence of a paragraph; a statement that gives the explicit or implied major topic of a passage and the specific way in which the passage is limited in content or reference.