Full Idea
We shall understand 'idealism' to be the doctrine that whatever exists, or at any rate whatever can be known to exist, must be in some sense mental.
Gist of Idea
'Idealism' says that everything which exists is in some sense mental
Source
Bertrand Russell (Problems of Philosophy [1912], Ch. 4)
Book Reference
Russell,Bertrand: 'The Problems of Philosophy' [OUP 1995], p.19
A Reaction
The interesting thing here is the phrase 'in some sense', which takes on a new light when we begin once against to take seriously ideas such as panpsychism. If the boundary between mind and brain is blurred, so is that between realism and idealism.