Full Idea
Whereas a sensation is by definition mental, a sense-datum might be mind-independent.
Gist of Idea
Sensations are mental, but sense-data could be mind-independent
Source
Godfrey Vesey (Collins Dictionary of Philosophy [1990], p.266)
Book Reference
Angeles,Peter A.: 'A Dictionary of Philosophy' [Harper 1981], p.266
A Reaction
This seems to be what Russell is getting at in 1912, as he clearly separates sense-data from sensations. Discussions of sense-data always assume they are mental, which may make them redundant - but so might making them physical.