initialism (acronym) of Palestinian Liberation Organization, by 1965.

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Definitions of PLO from WordNet

plo (n.)
a political movement uniting Palestinian Arabs in an effort to create an independent state of Palestine; when formed in 1964 it was a terrorist organization dominated by Yasser Arafat's al-Fatah; in 1968 Arafat became chairman; received recognition by the United Nations and by Arab states in 1974 as a government in exile; has played a largely political role since the creation of the Palestine National Authority;
Synonyms: Palestine Liberation Organization