sclerosis (sklə-rōʹsĭs) noun. plural scleroses (-sēz). 1. a. A thickening or hardening of a body part, as of an artery, especially from excessive formation of fibrous interstitial tissue. b. A disease characterized by this thickening or hardening. 2. Botany. The hardening of cells by the formation of a secondary wall and the deposition of lignin, sklerosis.