What is Non-refoulement?

Legal Definition
Non-refoulement is a principle of international law, which forbids the rendering of a true victim of persecution to his or her persecutor. Generally, the persecutor in mind is a state actor. It is a principle of both the customary and trucial law of nations.

In common usage, a push back is border control agents turning vessels around in international waters or expelling asylum seekers who have entered a border.

Non-refoulement is a key facet of refugee law, that concerns the protection of refugees from being returned or expelled to places where their lives or freedoms could be threatened. Unlike political asylum, which applies to those who can prove a well-grounded fear of persecution based on certain category of persons, non-refoulement refers to the generic repatriation of people, including refugees into war zones and other disaster locales.

It is debatable whether non-refoulement is a jus cogens (peremptory norm) of international law that forbids the expulsion of a person into a jurisdiction, usually his or her home-country, where that person might be again subjected to persecution.
-- Wikipedia