Desensitisation
Exposing a dog to the thing it finds difficult at a strength so low it does not react, then raising it in increments it barely notices.
Also called: desensitization · systematic desensitisation · gradual exposure
The word owners reach for instead is "getting him used to it", and the two are not the same. Getting a frightened dog used to something usually means repeatedly putting it in a situation it cannot cope with, which rehearses the panic rather than reducing it.
Real desensitisation is deliberately unspectacular. If your dog reacted during the session, the step was too big — that is information, not failure, and the next repetition happens further away or quieter.
This is the half that makes counter-conditioning possible, which is why the two are almost always described together.