Socialization window
The early period when a puppy or kitten accepts new things readily, and after which the same things start to be treated as threats.
It is a developmental stage, not a training deadline invented by trainers. During it, novelty is processed as interesting; after it closes, novelty is processed as suspicious, and the same exposure that would have been trivial at eight weeks takes months of careful work at eight months. The window opens and closes earlier in kittens than in puppies.
The practical tension is that it overlaps with the vaccination schedule, which is why the American Veterinary Society of Animal Behavior's position is that puppies should start socialisation classes before the course is finished, with sensible precautions. Quality matters more than volume: a calm, good experience of one child beats an overwhelming afternoon with ten.