Full Idea
Being is that which belongs to every conceivable, to every possible object of thought.
Gist of Idea
Being is what belongs to every possible object of thought
Source
Bertrand Russell (The Principles of Mathematics [1903]), quoted by Stephen Read - Thinking About Logic Ch.5
Book Reference
Read,Stephen: 'Thinking About Logic' [OUP 1995], p.131
A Reaction
I take Russell's (or anyone's) attempt to distinguish two different senses of the word 'being' or 'exist' to be an umitigated metaphysical disaster.