The Megali Idea (Greek: Μεγάλη Ιδέα, romanized: Megáli Idéa, lit. 'Great Idea') is a nationalist and irredentist concept that expresses the goal of reviving the Byzantine Empire, by establishing a Greek state, which would include the large Greek populations that were still under Ottoman rule after the end of the Greek War of Independence (1821–1829) and all the regions that had large Greek populations (parts of the southern Balkans, Anatolia and Cyprus).
The term appeared for the first time during the debates of Prime Minister Ioannis Kolettis with King Otto that preceded the promulgation of the 1844 constitution. It came to dominate foreign relations and played a significant role in domestic politics for much of the first century of Greek independence. The expression was new in 1844 but the concept had roots in the Greek popular psyche, which long had hopes of liberation from Ottoman rule and restoration of the Byzantine Empire.