Orlah

The prohibition on "orlah"-fruit (lit. "uncircumcised" fruit) is a command found in the Bible not to eat fruit produced by a tree during the first three years after planting. The Hebrew word "orlah" literally means "uncircumcised". This meaning is often footnoted in English translations:
In rabbinical writings the "orlah"-prohibition (Hebrew: איסור ערלה) is counted as one of the negative commandments among the rabbinical enumeration of 613 commandments. Outside of the land of Israel the prohibition also applies to a certain degree.
Bible.
Commentators generally assume that the law was good agricultural practice, and that early harvesting would conflict with careful cultivation and pruning during the first three years in order to insure later good harvests and allow maturing of the trees. Grape vines produce fruit in three to six years, almond trees produce some flower buds in the fourth year and some fruit in the fifth, and sources from the Ancient Near East suggest that a good crop of dates was expected in the fourth year. In discussing the commandment that the fruit could not actually be eaten until the fifth year, Rooker (2000) notes that in the Code of Hammurabi a tenant-gardener could not eat of the fruit of an orchard until the fifth year, when he shared the produce with the owner.
Rabbinical writings.
Mishnah.
The Mishna stipulates that Orlah fruit must be burnt to guarantee that no one benefits from them (Mishnah, Temurah 33b), and even a garment dyed by way of pigment derived from orlah is to be destroyed (Mishnah, Orlah 3:1).
Sifra.
The Sifra (to Leviticus 19:24) points out that the three year count begins on Rosh HaShana (the Jewish new year) and not "tree years" (the Jewish agricultural holiday of Tu Bishvat). Thus, the fruit of a tree only two years and 30 days old may not be considered forbidden.
Jerusalem Talmud.
The Jerusalem Talmud stipulates that "sofek orlah" (uncertainty if the product is indeed orlah) is permitted outside of the land of Israel (Orlah (Talmud) 20b). However, Rabbi Yochanan, in a letter sent to Rabbi Yehudah and quoted in the Babylonian Talmud, took a starkly stringent approach to the common practice of diasporic Jewry being overly lenient on "safek orlah";
Outside of the land of Israel.
Although the mitzvah of orlah is listed in the category of prohibitions pertaining to the Land of Israel alone ("מצווה שתלויה בארץ ישראל"), it is the only mitzvah of this category that applies outside of Israel as well - with certain leniencies (mishna, Kidushin ch. 1, Mishna 9). This application is forthcoming due to it being listed as a "Halakhah LeMoshe MiSinai" (a law given to Moses at Sinai, Sifra, 23:14). The tanna Rabbi Eliezer ben Hurcanus holds the opinion that the prohibition of orlah does not apply outside the land of Israel at all.
Questionable fruit.
Faced with an uncertainty as to whether an item is orlah (or a result of orlah usage such as dye, etc.), the mishna prescribes that such product is permitted for consumption so long as the actual removal of orlah product is not "seen" being picked (Mishnah, Orlah, ch. 3). 
The papaya fruit is a subject of rabbinic dispute. Some rabbinic authorities maintain that the papaya is not a tree, thus making it orlah-exempt, whereas most rule that the laws of orlah do apply to the papaya. Papain, (a "second crop" enzyme extracted from the papaya peel, used in beer, biscuits, and as a digestive aid) is likewise under rabbinic scrutiny as a dilution ratio of 200:1 (200 non-orlah fruit to 1 part orlah) is required to permit orlah, essentially prohibiting benefiting from this enzyme.
Use of the term "uncircumcised".
The term "uncircumcised" is explained by Shlomo Ephraim Luntschitz as meaning "hidden and sealed" and it alludes to the creation itself.
Practice in modern Israel.
In the State of Israel the laws of orlah have been observed literally. The Chief Rabbinate of Israel has allowed the sale of such fruit to non-Jews, but the usual policy is to destroy it.