How is the observable universe defined?
The part of the universe that it is theoretically possible to see.
Although we do not have the equipment to see all the observable universe, the observable universe is the part of the universe from which photons could have reached the Earth. Based on the age of the Universe, the observable Universe would be just under 14 billion light years from Earth. However, due to the fact that bodies are in motion and the Universe is expanding, the actual observable universe is much later than that. I am not positive how the actual number is arrived at but it is around 3 or 4 times bigger than 14 billion light years.
By contrast the furthest thing we can see with the naked eye is the Andromeda Galaxy, which is not even 5 million light years away. The furthest object that the Hubble Telescope has seen is 13.2 billion light years away.
So even with the best telescope we have we cannot see the entire observable Universe.