Tzedek meaning justice, has been a mark and aspiration of the Jewish people since its beginning. When God "decides" to share his plan to destroy Sodom with Abraham, He gives the reason that He has commanded Abraham and his descendants "to do tzedakah and mishpat", righteousness and justice. Therefore, God invites Abraham to take part in the process of moral decision-making about the fate of Sodom.
Tzedek is derived from the word zakah, meaning clear or pure. From that root, it comes to mean right or just. As a verb, it is used to mean to act on someone's behalf or to justify. One well-known cognate is tzadik, meaning a righteous or saintly person. It's an endearing part of Sephardi colloquial usage in Israel today that people call each other "tzadik", in the same way as you would say "mate" in English. Tzedakah, often translated as charity, is another relative. This is supposed to be such an ingrained Jewish trait that Maimomides says that there is cause to doubt the Jewish parentage of someone who does not give charity.
Today the word tzedek continues to express the aspiration to justice. It is the name of a UK charity that sends Jewish volunteers to development projects in the Third World. In Israel, Ma'agalei Tzedek is an organisation that brings Torah teachings to bear on contemporary problems of social justice and gives certification to restaurants and cafes that treat their workers fairly, which is displayed next to kashrut certificates.