The term 'ergo sum' is Latin for 'I think, therefore I am'. This phrase was famously used by the French philosopher Rene Descartes in his 1641 work 'Meditations on First Philosophy'. The crossword solution for this term is 'COGITO', which is also Latin for 'I think'. This phrase is closely associated with Descartes' philosophical views, and is often used to express the idea that one's own existence can be deduced from the fact that one is capable of thinking.