Yours, Etc.: Origins and Uses of 8 Sign-Offs
late 14c., "unaccented syllable or note," from Latin thesis "unaccented syllable in poetry," later "stressed part of a metrical foot," from Greek thesis "a proposition," also "downbeat" (in music), originally "a setting down or placing," from root of tithenai "to place, put, set," from PIE root *dhe- "to put, to do" (see factitious). Sense in logic of "a proposition, statement to be proved" is first recorded 1570s; that of "dissertation written by a candidate for a university degree" is from 1650s.
The central idea in a piece of writing, sometimes contained in a topic sentence.