Terumat hamaaser

In the Hebrew Bible, the tithe of the tithes (Hebrew: terumat ha-maaser) is a mitzvah (biblical requirement) for the recipient Levite to give to the priest a tenth (10%) of the tithe of produce that the former received from the Israelites. It applies only to agricultural produce grown in the Land of Israel.
This "tithe of the tithes" is a derivative of the tithe offering (Hebrew: "terumat ha-maaser") - a rabbinical Hebrew term based on the commandment in the Hebrew Bible to give a tithe "maaser" of 10% to the Levites. The first term, terumah, means offering. The term "tithe offering," "terumat ha-maaser", is alluded to in the Hebrew Bible text under the words "a tithe (tenth) of the tithe" (). 
Hebrew Bible.
The term "tithe" (Hebrew: "maaser") occurs 10 times in the Masoretic Text of the Hebrew Bible, in addition to the term "maasar hamaaser" (Hebrew: "tithe of the tithe", in the Septuagint Greek: "dekate tes dekates") which occurs once only in Nehemiah .
This offering is to be distinguished from the "offering tribute" (Hebrew: "terumat hamekhes") which Moses gave to God in Numbers .
Rabbinical interpretation.
The gift of "terumat ha-maaser" was generally not given by the Israelite directly to the priest. But it was given to a Levite, as the recipient of "maaser rishon" ("first tithe"; Hebrew: ), and then the Levite gave of ten percent of his "maaser rishon" gift directly to the "kohen". After the edict of Ezra, which directed "maaser rishon" to be given to the priest, it became the Kohen's responsibility to give one tenth of his "maaser rishon" gift to another priest of his choice. 
In the Hebrew Bible, the "terumah" ("offering") was regarded as a kind of sacred "korban" (also "offering"). It could be eaten only by Jewish priests and their families, had to be ritually pure, had to be eaten while in a state of ritual purity, and could not be taken out of the Land of Israel.
In modern times.
Orthodox Judaism requires taking "terumah" from produce grown in Israel, although in the absence of a Temple it is no longer given to the priests. In contemporary practice, most of the Terumah and various other biblical tithes (including first tithe and second tithe are first set aside. The "second tithe" ("maaser sheni") is then redeemed upon a coin of nominal value (not generally equal to the value of the produce). The coin and the unredeemable portion of the produce are then discarded in a manner that prevents their use. The reason for discarding in such a manner is that taking these tithes are sacred and must be preserved in a state of "purity" (Hebrew: "tahara") and eaten by a priest in a state of purity. In contemporary times, all people are considered to be defiled by a type of "impurity" (Hebrew: "tumah") which can only be purified through the ritual of the red heifer (Hebrew: "parah adumah"). It has not been possible to perform this sacrifice since the destruction of the Second Temple. Since it is forbidden to defile "terumah," the produce must be discarded in a manner commensurate with its holiness.