Full Idea
We mean by 'proposition' primarily a form of words which expresses what is either true or false. I say 'primarily' because I do not wish to exclude other than verbal symbols, or even mere thoughts if they have a symbolic character.
Gist of Idea
Propositions are mainly verbal expressions of true or false, and perhaps also symbolic thoughts
Source
Bertrand Russell (Introduction to Mathematical Philosophy [1919], XV)
Book Reference
Russell,Bertrand: 'Introduction to Mathematical Philosophy' [George Allen and Unwin 1975], p.155
A Reaction
I like the last bit, as I think of propositions as pre-verbal thoughts, and I am sympathetic to Fodor's 'language of thought' thesis, that there is a system of representations within the brain.