John Keats was an English Romantic poet who lived from 1795 to 1821. He is known for his works such as 'Ode to a Nightingale' and 'Ode on a Grecian URN'. The crossword solution 'Keats' 'Foster-Child of Silence and Slow Time' is one' is a reference to the poem 'Ode on a Grecian URN'. In this poem, Keats uses the metaphor of an ancient URN to explore the idea of time and mortality. He describes the URN as a 'foster-child of silence and slow time', meaning that it has been around for so long that it has seen many things, but it will never age or change.