A savant is an Allomancer who has flared their metal for so long that it has changed their physiology.
With most metals the effects are quite small. Seekers for example often become savants without being aware of it. This grants them a boost to the range of their abilities. However, some metals warp the body of savants much more. Tin savants for example experience such magnification of their senses that they can become incapacitated by bright light or loud sounds. Pewter Mistings often die before becoming savants[1] as they feel neither pain nor exhaustion to any great extent and therefore may ignore wounds which later prove fatal.
Spook is the most obvious example of a Savant in the Mistborn trilogy. He flares his tin for so long that his body comes to a point where it literally cannot survive without the tin. Any light becomes too bright for him, and he takes to wearing a blindfold at all times. This does not hamper him, however, because his other senses have become so attuned that he can feel and hear well enough to make up for the lack of vision.