define observation in science
But to see it more clearly: scientific observation is to view  the movements, and effects of those movements, or to view any  situation, and the effect of it.   Example: I saw a fish swim though the water. So based on what I  observed, I ask questions, like, how did the fish swim though the  water.
The definition of observational science is a field of science where controlled observations cannot be done in order to study causes and effects. Scientific studies are simply done through the observation of nature taking its course and recording the findings over time. Astronomy.