The Tzadikim Nistarim (Hebrew: צַדִיקִים נִסתָּרים, hidden righteous ones) or Lamed Vav Tzadikim (Hebrew: ל"ו צַדִיקִים, 36 righteous ones), often abbreviated to Lamed Vav(niks), refers to 36 Righteous people, a notion rooted within the more mystical dimensions of Judaism. The singular form is Tzadik Nistar (Hebrew: צַדִיק נִסתָר).
Read more about Tzadikim Nistarim: Origins, Their Purpose, Lamedvavniks