The 1951 Refugee Convention, negotiated after World War II, defines a refugee as a person who, “owing to a well-founded fear of being persecuted for reasons of race, religion, nationality, membership of a particular social group or political opinion, is outside the country of his nationality, and is unable to, or owing to such fear, is unwilling to avail himself of the protection of that country.”

Among those crossing the Mediterranean in the first half of 2015, the greatest numbers came from Syria, Afghanistan or Eritrea. Syrians are widely presumed to be refugees because of the civil war there, according to the United Nations refugee agency. Many Afghans have been able to make the case that they are fleeing conflict, the agency added, and Eritreans can generally argue that they would face political persecution at home in Eritrea, which is ruled by one of the world’s most repressive regimes.

Q. What does the distinction mean for European countries?

A. Refugees are entitled to basic protections under the 1951 convention and other international agreements. Once in Europe, refugees can apply for political asylum or another protected status, sometimes temporary. By law, refugees cannot be sent back to countries where their lives would be in danger. “One of the most fundamental principles laid down in international law is that refugees should not be expelled or returned to situations where their life and freedom would be under threat,” the refugee agency said in a statement on Thursday.