Full Idea
A fact is the kind of thing that makes a proposition true or false, …and it is the sort of thing that is expressed by a whole sentence, not by a single name like 'Socrates'.
Gist of Idea
Facts make propositions true or false, and are expressed by whole sentences
Source
Bertrand Russell (The Philosophy of Logical Atomism [1918], §I)
Book Reference
Russell,Bertrand: 'Russell's Logical Atomism', ed/tr. Pears,David [Fontana 1972], p.36
A Reaction
It is important to note a point here which I consider vital - that Russell keeps the idea of a fact quite distinct from the language in which it is expressed. Facts are a 'sort of thing', of the kind which are now referred to as 'truth-makers'.