What is stimulus generalization?

Stimulus generalization is a concept in psychology where, if you're exposed to a certain stimulus and are conditioned to react in a certain way to it, you will also react in a very similar way to stimuli that look or seem to be very similar to the original stimulus.

An example of this would be if you were bitten by a dog and are conditioned to being afraid of being bitten by a dog, you will be also likely afraid of similar dogs when you see them in different situations.