mathematical definition of prism
1. countable noun. A prism is a block of clear glass or plastic which separates the light passing through it into different colours. 2. countable noun [usually singular , NOUN of noun] If you see something through a prism of something such as time or memory, your idea of it is affected by that thing. [literary] Through the smoky prism of time, I could just barely make out my father as a young man.