(Mathematical Logic:)
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 xn+yn!=zn". "x2=2*y2" is not a proposition, as the variables x,y are free. "2+2", "2+, ")))x(+" aren't even formulae, hence not propositions.