Can someone tell me the definition of observation?
It is of course a very subtle issue. First of all, "observation" as such doesn't, I think, have any meaning. You have to observe *something*, and that something, in a physics setting, is "an outcome of experiment" (which can go from very sophisticated to trivial).
I would then say that an outcome of experiment is observed, if (at least in principle) it is possible to write down that outcome on a sheet of paper which can be send to a newspaper, and any action/setup/experiment that performs this, is an observation.
I would say that any quantum system can be said to have been observed if it interacts with an outside agency.
In the quantum context, I will say that an observation result is simply a logical statement that is true (e.g. the particle is observed at position x <=> "the result of the observation is x" is true).
Now you have to explain what "interaction" and "outside agency" mean. You've just doubled your workload.
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