What bookdealers really mean. A dictionary by Tom Congalton and Dan Gregory (Between the Covers), with additions from other sources
Darkening to paper and inks, often from exposure to sunlight, but sometimes resulting simply from prolonged exposure to ambient light. Books kept on a shelf for several decades will usually exhibit tanning over time, even if they are never touched. Some dealers use more pejorative terms such as darkened or browned, but we prefer the more cheery implications of the book being "tanned," as though it had spent a happy and contented summer lolling away on the beach drinking margaritas.