In an answer regarding musical themes, Beethoven's Fifth Symphony is used to illustrate how themes can comprise material beyond the melodic. However, I typically hear "theme" used to refer to longer, melodic ideas, and "motif" used for sub-parts of a theme that provide compositional material.
Yet while I hear Beethoven 5 (first movement) as clearly having two themes, the first theme is certainly not a theme in the above sense. The linked answer discusses a small motif as "the theme". Are the terms interchangeable? Are they sometimes the same and sometimes different according to the context of their use? Or do they have clearly defined, distinct meanings? (In other words, is it wrong to describe Beethoven 5's opening motif as a theme?)