What is "bletting," a term used in relation to medlar fruits?

Simple definition of bletting
Well, at its simplest it's a stage of fruit development in-between ripening and rotting. It describes when a fruit has fully ripened, has started to break down, but is not quite rotting yet. There's a bit of semantics at play here, strictly speaking bletting is actually the early stages of rotting, but before the fruit goes bad.

Medlar fruits, Mespilus germanica, are inedible when hard and can only be eaten once they have been allowed to 'blet'. They are picked in late autumn and stored in a cool, dry place until about two weeks later they soften, become mushy and turn a darker brown. This process is known as 'bletting'. The flesh now smells like ripe apples, tastes like honey and has a texture of custard. Yum. This is one of those fruits that you don't see unless you grow them yourself.