APIKORIS:
The Greek term άίρεσις originally denoted "division," "sect," "religious" or "philosophical party," and is applied by Josephus to the three Jewish sects: Sadducees, Pharisees, and Essenes. The specific rabbinical term for heresies, or religious divisions due to an unlawful spirit, is "מינים/minim" (lit. "kinds {of belief}"; the singular "מין/min," (literally, "a kind") for "heretic" or "Gnostic," is coined idiomatically, like "goy" and "'am ha-aretz";). An alternative term, אפיקורוס (Apiqoros/Apikoros), which may have a different status[2], and is often used for apostates who turned towards agnosticism or atheism, is derived from Epicurus and the philosophy of Epicureanism.
The law "You shall not cut yourselves" (Deuteronomy 14:1) is interpreted by the Rabbis: "You shall not form divisions, but shall form one bond." (Source: Talmud Yevamot 13a, Midrash Sifre on Deuteronomy 96)
Besides the term "min" for "heretic," the Talmud uses the words "Hitsonim" (outsiders), "apikoros," and "kofer ba-Torah" (R. H. 17a), or "kofeir ba-'ikar[3]" (he who denies the fundamentals of faith; Pes. xxiv. 168b). Similar statuses may apply to some of those those branded "poresh mi-darke tsibbur" (he who deviates from the customs of the community; Tosef., Sanh. xiii. 5; R. H. 17a). Some authorities opine that all of these are consigned to Gehinnom for all eternity (Tosef., Sanh. l.c.; comp. ib. xii. 9, apparently belonging to xiii. 5: "He who casts off the yoke [of the Law], and he who severs the Abrahamic covenant; he who interprets the Torah against the halakic tradition, and he who pronounces in full the Ineffable Name—all these have no share in the world to come,") or possibly have no afterlife at all[4].
The Mishnah says the following have no share in the world to come: "He who denies that the Torah is divinely revealed, and the apiḳoros." R. Akiba says, "also he who reads heretical books". This is explained in the Talmud (Sanh. 100b) to mean "sifre Ẓeduḳim" (Sadducean writings); but this is an alteration by the censor of "sifre ha-Minim" (books of the Gnostics or Heretics). The Biblical version, "That ye seek not after your own heart" (Num. xv. 39), is explained (Sifre, Num. 115; Ber. 12b) as "Ye shall not turn to heretic views ["minut"] which lead your heart away from G-d" (see Maimonides, "Yad," 'Akkum, ii. 3).
In summarizing the Talmudic statements concerning heretics in Sanh. 90-103, Maimonides ("Yad," Teshubah, iii. 6-8) says:
"The following have no share in the world to come, but are cut off, and perish, and receive their punishment for all time for their great sin: the minim, the apiḳoresim, they that deny the belief in the Torah, they that deny the belief in resurrection of the dead and in the coming of the Redeemer, the apostates, they that lead many to sin, they that turn away from the ways of the [Jewish] community... Five are called 'minim': (1) he who says there is no G-d and the world has no guide; (2) he who says the world has more than one guide; (3) he who ascribes to the Lord of the Universe a body and a figure; (4) he who says that G-d was not alone and Creator of all things at the world's beginning; (5) he who worships some star or constellation as an intermediating power between himself and the Lord of the World.
"The following three classes are called 'apiḳoresim': (1) he who says there was no prophecy nor was there any wisdom that came from G-d and which was attained by the heart of man; (2) he who denies the prophetic power of Moses our master; (3) he who says that G-d has no knowledge concerning the doings of men.
"The following three are called 'koferim ba-Torah': (1) he who says the Torah is not from God: he is a kofer even if he says a single verse or letter thereof was said by Moses of his own accord; (2) he who denies the traditional interpretation of the Torah and opposes those authorities who declare it to be tradition, as did Zadok and Boethus; and (3) he who says, as do the Nazarenes and the Mohammedans, that the Lord has given a new dispensation instead of the old, and that he has abolished the Law, though it was originally divine."