Thickened stratum corneum of an epithelium, predominantly of the epidermis but also of an infundibulum or an eccrine duct. The thickening of the stratum corneum can be the result of proliferative hyperkeratosis ( lichen planus, clavus, lupus erythematosus) or retention hyperkeratosis (vulgar ichthyosis). The hyperkeratosis can be regular, orthokeratotic or pathological, parakeratotic.