Key Indexed Directory
Here is the real definition.
A KID provides an efficient mutable partial map from keys to integers.
To do this it employs a closely held key to a kernel object, KEY_BITS, that reveals the unprepared bits of keys.
Seeing the bits allows the KID to build a b-tree which can be efficiently consulted.
The KID is trusted to limit its use of KEY_BITS to providing only its specified functions.