The Aramaic term segan (סגן) or segan hakohanim in the Talmud is the title for a Jewish Priest

Kohen

A Kohen is the Hebrew word for priest. Jewish Kohens are traditionally believed and halachically required to be of direct patrilineal descent from the Biblical Aaron....

 of the Temple in Jerusalem

Temple in Jerusalem

The Temple in Jerusalem or Holy Temple , refers to one of a series of structures which were historically located on the Temple Mount in the Old City of Jerusalem, the current site of the Dome of the Rock. Historically, these successive temples stood at this location and functioned as the centre of...

.

Hebrew Bible

The form segan is Aramaic (סְגַן), appearing 5 times in the Hebrew Bible

Hebrew Bible

The Hebrew Bible is a term used by biblical scholars outside of Judaism to refer to the Tanakh , a canonical collection of Jewish texts, and the common textual antecedent of the several canonical editions of the Christian Old Testament...

 in the Aramaic sections of the Book of Daniel to refer to officers of the Babylonian government. The Hebrew form sagan (סָגָן) occurs a further 17 times in Nehemiah

Nehemiah

Nehemiah ]]," Standard Hebrew Nəḥemya, Tiberian Hebrew Nəḥemyāh) is the central figure of the Book of Nehemiah, which describes his work rebuilding Jerusalem and purifying the Jewish community. He was the son of Hachaliah, Nehemiah ]]," Standard Hebrew Nəḥemya, Tiberian Hebrew Nəḥemyāh) is the...

 and elsewhere, again to refer to officials of the Babylonian rulers.

Talmud

According to the Talmud the deputy was appointed to the position of the segan ha-kohanim with the responsibility of overseeing the actions of the work of the Temple's priests' staff, as well as a stand-in position, ready to take the role of High Priest in case he will be found unfit to serve the holy work on the temple, and thus, the Segan was only second to the High Priest, as Rabbi Hanina Segan ha-Kohanim (40 – 80 CE) attests:
Many times the title commonly appears on the classical texts as ha-Segan ("the Deputy"), instead of the full title of Segan ha-Kohanim, for example on the Mishnah

Mishnah

The Mishnah or Mishna is the first major written redaction of the Jewish oral traditions called the "Oral Torah". It is also the first major work of Rabbinic Judaism. It was redacted c...

, in an halakha

Halakha

Halakha — also transliterated Halocho , or Halacha — is the collective body of Jewish law, including biblical law and later talmudic and rabbinic law, as well as customs and traditions.Judaism classically draws no distinction in its laws between religious and ostensibly non-religious life; Jewish...

 that deals with the work of the High Preist on Yom Kippur

Yom Kippur

Yom Kippur , also known as Day of Atonement, is the holiest and most solemn day of the year for the Jews. Its central themes are atonement and repentance. Jews traditionally observe this holy day with a 25-hour period of fasting and intensive prayer, often spending most of the day in synagogue...

:
One can also note the importance given to the matter in the ritual ceremony of "Nichum Aveilim" (consoling the mourners of the deceased), in which the High Priest takes part in:
Two out of three most prominent Segans are noted on the Talmud

Talmud

The Talmud is a central text of mainstream Judaism. It takes the form of a record of rabbinic discussions pertaining to Jewish law, ethics, philosophy, customs and history....

 and on Josephus Flavius' work: Hanina Segan ha-Kohanim, and Eleazar ben Hanania (son of Hananiah b. Hezekiah b. Garon

Hananiah b. Hezekiah b. Garon

Hananiah ben Hezekiah ben Garon was a Jewish Tanna sage, contemporary of House of Shammai and House of Hillel era. He is being recounted as one that after several sages had weighed in, to make Genizah on the Book of Ezekiel, he made aliyah to the Land of Israel and took 300 barrels of oil along...

who served as High Priest).