Bilateral is negotiating with a partner. Unilateralism is attempting to solve the conflict via your own actions without ANYONE's input e.g. Israel's attempts at "facts on the ground", which are the very definition of unilateral. An appeal to the UN is not unilateralism i.e. it is an appeal to an outside arbiter, it is not an attempt by the Palestinians to solve this conflict on their own.
U.S. to defend Syrian rebels with airpower, including from Assad (Reuters)