Though you pronounce it duh-LIN-ee-ate, there is a "line" in the middle of delineate. This might help you remember that to delineate is to outline and define something in detail or with an actual marking of lines and boundaries.
When you create an outline for a paper it usually summarizes what you will detail later. You delineate the sections, or mark the heading lines, and when you write the details, you delineate the subject of each heading. So, to delineate is both to mark lines and to fill in the lines. Using a fence to divide properties or a carpet to claim your side of the bedroom also is a way to delineate, or mark, physical boundaries.