(vi-sis'i-tOOd", -tyOOd"), —n. 1. a change or variation occurring in the course of something. 2. interchange or alternation, as of states or things. 3. vicissitudes, successive, alternating, or changing phases or conditions, as of life or fortune; ups and downs: They remained friends through the vicissitudes of 40 years. 4. regular change or succession of one state or thing to another. 5. change; mutation; mutability.