What defines an observer in quantum mechanics?

The way I understand it, electrons within atomic shells are in 'superpositions' until they are observed. But what does it mean for an electron to be 'observed'? There's nothing fundamental about a human's eye that would cause an electron's state to change. Could a cat observe it? An act? A bacterium? Why couldn't an electron 'observe' itself?

I feel I'm missing something obvious here so sorry if this is a stupid question.