Full Idea
In 'I met a unicorn' the four words together make a significant proposition, and the word 'unicorn' is significant, …but the two words 'a unicorn' do not form a group having a meaning of its own. It is an indefinite description describing nothing.
Gist of Idea
'I met a unicorn' is meaningful, and so is 'unicorn', but 'a unicorn' is not
Source
Bertrand Russell (Introduction to Mathematical Philosophy [1919], XVI)
Book Reference
Russell,Bertrand: 'Introduction to Mathematical Philosophy' [George Allen and Unwin 1975], p.170