Impression refers to the mental bonding that occurs between a Dragon, Fire lizard or Wher and a human. For fire lizards and whers, the bonding is almost a reflexive, instinctive thing and it quite often occurs with the first person who offers food to the newborn. When a fire lizard or wher is Impressed, it can give its trainer or owner impressions about what it is feeling, and it has the ability to project its thoughts to a limited extent.
However, for a Pernese dragon, impression is not a choice, but a necessity, as Impression goes far deeper for a dragon. It has been said that a dragon is incomplete without its lifemate, and cannot survive if it does not find one. There have even been cases when a newly-hatched dragon cannot find a suitable match and goes between.
The emotions, thoughts, desires, wants and needs of both dragon and human partners are known to each other. The dragon knows his or her name, and so tells it to the human counterpart when the dragon introduces itself.
For a dragon and a human, Impression is more than merely the melding of two minds together. They literally become one being with two bodies, their souls intertwined to the extent that there is no way for them to tell where dragon begins and human ends. Thus, it surpasses the familiar concept of telepathy.
Though a human can live, although shattered and incomplete, with the loss of his or her dragon, if the human partner dies the dragon will instantly go between as it cannot comprehend or fathom life without its human counterpart. In the case of Gold dragons with unhatched eggs, the maternal instinct can prevent the dragon from committing suicide until her eggs have safely hatched and the hatchlings themselves have impressed.
Impression being necessary for dragons was the deliberate work of geneticist Kitti Ping Yung, who recognized that introducing large, flying carnivores that could think for themselves into the planet's ecosystem, without a definitive control, would be disastrous for both humans and all other species on Pern. A dragon growing up without human guidance, or outliving their rider, could potentially become extremely dangerous.