Something that's observed is seen or noted, especially as part of a scientific investigation. An observed side effect of a medication is one that's been scientifically proven.
Scientific inquiry includes trials and tests that are repeated many times — to prove something, a scientist has to actually see it, or observe it. When you're talking about science, something that's observed has been officially determined to be true. Observed and observe come from the Latin root observare, which means "note or regard," but also "keep safe or protect."