The term gadol hador refers to the "great/est (one of) the generation" denoting one rabbi who is presumed to be even greater than the others. Other variations of the term are Gadol Yisrael or a Gadol BeYisrael (plural: Gedolei Yisrael), meaning "great one of the Jewish people".
A similar title is Rashkebahag, which is an acronym for "Rabbon shel kol bnei hagolah" (The sage and teacher of the entire Jewish diaspora). Another term is Manhig Yisroel (plural: Manhigei Yisroel), literally "leader of Israel".
The title gadol hador is usually only given to one Jewish Sage at a time, while the title "Rashkebahag" can be given to a few, and the term Gedolei Yisrael collectively refers to all leading rabbis in the Haredi community.
Most often a gadol functions as a rosh yeshiva (the head of a yeshiva Talmudical school), and can be a Hasidic Rebbe. A gadol is quite often also a posek (a decisor of Halakha - Jewish law) and may be the author of rabbinic literature and responsa.
Rabbi Chaim Epstein has been quoted as saying:
In Hebrew halachic texts, gadol is also used as a term for a Jewish boy who turns thirteen, and is viewed as an adult regarding to his obligation to practice the 613 commandments. This is the age of Bar Mitzvah. When a Jewish girl reaches the age of twelve, according to Jewish law, she is called a gedolah (the feminine form of gadol).
In modern Hebrew, gadol as slang is used as an interjection to mean something is extremely cool, out of this world, superb, awesome, absurdly funny or hilarious. For example, upon hearing a funny joke one might interject "Gadol!"
In English writing, the transliterated word "gadol" generally refers to a prominent rabbi.