Egregoros

@sun an observer is the object or energy with which you perturb a physical property of an object or phenomenon

you need to be aware of heisenberg's uncertainty principle to understand what an observer is

he proved that it is impossible to accurately measure both the position and velocity of a particle or phenomenon. In order to measure one, you must necessarily interfere with the other and your interference will effect the thing you are measuring.

For example, if you want to see a can of Monster energy drink, you need to bounce light off the can and receive it with your eyes. This alters the state of the can (it might move it slightly, or warm it up, etc). In this case the observer would be the light itself

if you were probing a surface with a hard diamond tipped probe to measure its flatness, the observer would be the tip of the diamond-tipped probe

if you were setting up Muon detectors to find voids in the Great Pyramid, the observers would be the muons

the process of altering the state of the object or phenomena to measure its property is called observation