Tikkun means repair, restoration, setting right. In Lurianic Kabbalah it names the third moment of a cosmic drama: after the withdrawal of tzimtzum and the shattering of the vessels, the sparks of divine light scattered and held captive in the husks are to be raised and restored to their place. What does the raising is the performance of the commandments with proper intention. The result is a theology in which human action has genuine cosmic consequence, the restoration of the divine order is not accomplished by God alone, and Israel's ordinary observance is assigned a share in it.
The phrase tikkun olam has a longer and quite separate history. It appears in the Mishnah in a legal sense, mipnei tikkun ha-olam, "for the sake of the ordering of the world", as the stated justification for rabbinic enactments adjusting the law so that society continues to function, the prosbul and a series of rules governing divorce documents among them (Gittin 4:2-5:3). It appears again in the Aleinu prayer, in the phrase le-takken olam be-malkhut Shaddai, to perfect or establish the world under the sovereignty of the Almighty, which in its liturgical context is an eschatological hope for the universal recognition of God, not a programme of action.
From the 1950s, and rapidly from the 1970s, tikkun olam was taken up in North American Judaism, especially in Reform, Conservative and Reconstructionist settings, as the standard term for social justice understood as a religious obligation. The usage is now thoroughly established in synagogue life, youth movements and communal organisations.
It is also contested. Some Orthodox writers, and some historians who have traced the phrase, argue that the modern usage is discontinuous with both the halakhic and the kabbalistic senses, and that it risks substituting a general political programme for the specific content of Jewish obligation. Defenders reply that the prophetic and legal traditions ground a social mandate abundantly whatever this particular phrase once meant, that religious vocabularies are always redeployed, and that the objection is often to the politics rather than to the philology. The argument is live and is conducted by serious people on both sides.