Full Idea
We offer the following tentative definition: The matter of a given thing is the limit of its appearances as their distance from the thing diminishes.
Gist of Idea
Matter is the limit of appearances as distance from the object diminishes
Source
Bertrand Russell (The Relation of Sense-Data to Physics [1914], §IX)
Book Reference
Russell,Bertrand: 'Mysticism and Logic' [Unwin 1989], p.158
A Reaction
This strikes me as empiricism gone mad. Russell is famous for being a 'realist', but you would hardly know it at this point. Personally I put emphasis on 'best explanation', which fairly simply delivers most of our commonsense understandings of reality.