S0625700 (spĕn′sə-rĭz′əm) also Spen·ce·ri·an·ism(spĕn-sîr′ē-ə-nĭz′əm)n. The system of thought developed by Herbert Spencer, setting forth the idea that evolution is the passage from the simple, indefinite, and incoherent to the complex, definite, and coherent.