The part of the entire universe that is (in principle) visible to an observer at a given time and place, i.e. that lies within the observer's particle horizon.
Origin
1930s; earliest use found in Science.
Definition of observable universe in US English:
observable universe
noun
Astronomy
The part of the entire universe that is (in principle) visible to an observer at a given time and place, i.e. that lies within the observer's particle horizon.