Literally the Great of the Generation, the term gedol hador refers to the intellectual giants of the religious world. Rabbi Joseph B. Soloveitchik was a gedol hador. Rabbi Ovadia Yosef, leader of the Sephardic community, is a living gedol hador.
Ideally, a gedol hador has the ability to unite all factions of the Jewish people through the confidence that his (or her there has yet to be a female gedolat hador, but with the advancement of womens learning in Israel and abroad, it may only a matter of time) decisions combine incomparable intellectual acumen, knowledge and the humility of fear of Heaven (yirat Shamayim).
The expression dates back to Talmudic times. Hillel referred to Shmayah and Avtalyon as gedolei hador. Interestingly, the Jerusalem Talmud preferred the expression gedol olam, which had often been translated as the Great of the World.
However, the scholar Shama Friedman points out that Gedol Olam actually means the Great of all Time. For olam can mean both world and forever, as in for His kindness lasts forever, ki leolam chasdo (Psalms 136:1).
While gedol olam is the bigger compliment, gedol hador implies a readiness to lead the generation.
Every generation needs its own gedolim who combine total command of the traditional Torah sources with an awareness of the ever-new and unique challenges of the time.