A well formed formula (wff) of predicate calculus with no free variables. Propositions are the

*sentences*(or statements) of logic: a proposition is something to which a truth value can be attributed.

Examples of propositions are
"2+2=4", "2+2=5", "for all positive integers x,y,z,n, if n>2 then x^{n}+y^{n}!=z^{n}". "x^{2}=2*y^{2}" is *not* a proposition, as the variables x,y are free. "2+2", "2+, ")))x(+" aren't even formulae, hence not propositions.