1. A remark which reveals important plot elements from books or movies, thus denying the reader (of the article) the proper suspense when reading the book or watching the movie.
2. Any remark which telegraphs the solution of a problem or puzzle, thus denying the reader the pleasure of working out the correct answer (see also interesting). Either sense readily forms compounds like "total spoiler", "quasi-spoiler" and even "pseudo-spoiler".
By convention, Usenet news articles which are spoilers in either sense should contain the word "spoiler" in the Subject: line, or guarantee via various tricks that the answer appears only after several screens-full of warning, or conceal the sensitive information via rot13, or some combination of these techniques.