Full Idea
Let us give the name 'sense-data' to the things that are immediately known in sensation: such things as colours, sounds, smells, hardnesses, roughnesses, and so on.
Gist of Idea
'Sense-data' are what are immediately known in sensation, such as colours or roughnesses
Source
Bertrand Russell (Problems of Philosophy [1912], Ch. 1)
Book Reference
Russell,Bertrand: 'The Problems of Philosophy' [OUP 1995], p.4
A Reaction
This idea gradually became notorious, because it seems to create a new ontological category unnecessarily, and it creates problems, such as how the intermediary interacts with us and with things. Are sense-data totally non-conceptual?