"god" comes from the Proto-Indo-European ghut-, meaning "that which is invoked". So a "god" is anything which one invokes for power.

"deity" or "divine" comes from the Proto-Indo-European dyeu-, meaning "to shine". So a "deity" is a being or object which "shines" "light" into the world. Light being Truth being the force behind existential assertions (The fuel of the Wheel).

So, the primary difference between a god and a deity is that a god requires a third agent to call upon its power, while a deity is defined simply through its baseline existential patterns (those patterns being that of a star, or a sun).