According to the Hebrew Bible, the kaporet (Hebrew: כַּפֹּרֶת kapōreṯ) or mercy seat was the gold lid placed on the Ark of the Covenant, with two cherubim at the ends to cover and create the space in which Yahweh appeared and dwelled. This was connected with the rituals of the Day of Atonement. The term also appears in later Jewish sources, and twice in the New Testament, from where it has significance in Christian theology.

The etymology of kaporet (Hebrew: הַכַּפֹּֽרֶת) is unclear. Baruch J. Schwartz in The Oxford Dictionary of the Jewish Religion states that "some translate simply 'cover'", whilst others posit a different Hebrew or foreign origin.

According to the biblical account (Exodus 25:19; 37:6), the cover was made from pure gold and was the same width and breadth as the ark beneath it, 2.5 cubits long and 1.5 cubits wide. Two golden cherubim were placed at each end of the cover facing one another and the mercy seat, with their wings spread to enclose the mercy seat (Exodus 25:18–21). The cherubim formed a seat for God (1 Samuel 4:4). The ark and mercy seat were kept inside the Holy of Holies, the temple's innermost sanctuary which was separated from the other parts of the temple by a thick curtain (parochet).

The Holy of Holies could be entered only by the high priest on the Day of Atonement. The high priest sprinkled the blood of a sacrificial bull onto the mercy seat as an atonement for the sins of the people of Israel.

After the destruction of the Second Temple, just as the Torah scroll was contained in a Torah ark (Aron HaKodesh, "Holy ark") in synagogues, so also the term kaporet was applied to the valance of the parochet (Hebrew: פרוכת‎ "curtain") on this ark.

In the Hellenistic Jewish Septuagint the term was rendered hilastērion (ἱλαστήριον, "thing that atones"), following the secondary meaning of the Hebrew root verb "cover" (כָּפַר‎ kaphar) in pi'el and pu'al as "to cover sins", "to atone" found also in kippurim. Hilastērion is relatively rare in classical Greek and appears largely in late writings to reference a sacrifice to appease the wrath of a deity. The term in the Septuagint was translated in the Latin Vulgate Bible with the word propitiatorium from which we get our English word propitiation.

Hilastērion (see in section above) is found twice in the New Testament: Romans 3:25 and Hebrews 9:5. In the passage in Romans the term is typically translated "propitiation" or "sacrifice of atonement", whereas in the passage in Hebrews the term is typically translated "mercy seat", the traditional term for the gold lid on the Ark of the Covenant. The difference in translation is explained by the different contexts. In Romans the context is the sacrificial death of Christ, whereas in the Hebrew passage the context is a description of the Holy of Holies and its contents. The Epistle to the Hebrews portrays the role of the mercy seat during Yom Kippur Day of Atonement as a prefiguration of the Passion of Christ, which was a greater atonement, and the formation of a New Covenant (Hebrews 9:3–15). The Yom Kippur ritual was a shadow of things to come (Hebrews 10:1). The continual sacrifice for sin under the Mosaic covenant became obsolete following the once-for-all sacrificial death of Christ.

The first English Bible, translated from Latin 1382, renders the term a propiciatory following the Vulgate propitiatorium, and in the first occurrence, Exodus 25:17, also inserts an unbracketed gloss "that is a table hiling the ark" – hiling is Middle English for "covering".