The Collaborative International Dictionary of English v.0.48:

Insole \In"sole`\, n.
   The inside sole of a boot or shoe; also, a loose, thin strip
   of leather, felt, etc., placed inside the shoe for warmth or
   ease.
   [1913 Webster]

WordNet (r) 3.0 (2006):

insole
    n 1: the inner sole of a shoe or boot where the foot rests [syn:
         insole, innersole]