3.31 Every part of a proposition that characterizes its sense I call an expression (a symbol).
(The proposition itself is an expression.)
The expression is all that is essential for the sense of a proposition that propositions can have in common with each other.
An expression marks a form and a content.
OK. More definitions. And how do expressions differ from propositions? Well, a proposition is an expression, so there cannot be much difference. An expression is also any part of a proposition that is proposition-like too, but this might also be called a proposition surely. So we don’t seem to have gained much here.
Black (p. 123): “Wittgenstein is not defining this sense of ‘symbol’ but merely adding that an expression is a symbol.”