The term idionymon (Greek: ιδιώνυμο, "special illegal act", delictum sui generis) was defined by a Greek law, voted in 1929 (Law 4229), after being introduced by the Eleftherios Venizelos government. It was a law "concerning safety measures for the social establishment and protection of the freedom". It was aiming to penalize the "insurrectional" ideas and in particular to fire prosecution against communists, anarchists and enforce repression against the unionist mobilizations.
The law prescribed a penalty for more than six months imprisonment for everyone "who tries to apply ideas that have as an obvious target the violent overthrow of the current social system, or who acts in propagandizing their application...". It was the first legal measure against the Communist Party of Greece and initiated a series of urgent legislation acts, established by the Greek state against the left wing. The idionymon accusation was enough for the government to ban and dissolve most of the workers organizations at the end of 1930. According to the law, the strike was no more a means for promoting political requests, but a disturbance of the social peace and unionism was an illegal act.
The establishment of the idionymon resulted from the perceived need to defend the gains of the (partially completed) reformation towards a bourgeois democracy in Greece, mainly led by Venizelos' Liberal party. The Second Hellenic Republic was inherently unstable, and furthermore, by the late 1920s, the old political dualism between Venizelists and Royalists was beginning to be threatened by agitation in the emerging working class. The defeat of the Asia Minor Campaign in 1922, which resulted in the arrival of over 1.5 million refugees, mostly impoverished and living in atrocious conditions, resulted in the emergence, for the first time, of a large urban working class, to whom the radical/communist ideas of the Russian Revolution might appeal.
Two of the leaders of the liberal opposition in Parliament, Alexandros Papanastasiou and Georgios Kafantaris, had expressed strong disagreement during the vote. It is remarkable that Eleftherios Venizelos rejected Papanastasiou's proposal to use idionymon not only against communists, but also against fascists, although it is perhaps understandable in light of their low political presence in Greece (relative to socialists) and Venizelos' ongoing diplomatic rapprochement with Fascist Italy.
Following the establishment of the dictatorial "4th of August Regime" in 1936, the idionymon formed the basis for Compulsory Law 117/1936, which featured harsher provisions, including five-year jail terms and internal exile.