Synopsis
Unfortunately, many agents consider a synopsis and an outline to be interchangeable. They shouldn't be. A true outline is much, much longer than a synopsis, up to forty or more pages, is usually done chapter by chapter, from one paragraph to one full page for each chapter, and is sent along with three chapters. But when many agents ask for an outline, they mean a synopsis.
Very few even know what a real outline is these days. Generally, pro writers are the wrong ones to ask because I doubt any of us write real outlines. Like Robert J. Sawyer, we write each the same way, maybe one before the novel is written, and a more accurate one after the novel is written, though I think Sawyer reverses the two, compared to the way most of us think of them.
For most pro writers, a synopsis is just an excuse for the publisher to offer a contract. It's written before the novel is, and the novel itself may be almost nothing like what was described in the synopsis.
For new writers, however, the query is used to convince an agent to read a partial or a full manuscript. The synopsis is then often used to convince an editor to read a partial or a full manuscript.