vice·roy / ˈvīsˌroi/ • n. 1. a ruler exercising authority in a colony on behalf of a sovereign. 2. a migratory orange and black butterfly (Limenitis archippus) that closely resembles the monarch but is typically somewhat smaller. The caterpillar feeds on willow leaves, and the adult mimics the unpalatable monarch. DERIVATIVES: vice·roy·al / ˌvīsˈroi-əl/ adj. vice·roy·ship / -ˌship/ n.