BDB classifies כִּפֻּרִים tersely as a "noun plural abstract atonement, only in P": it is a qittul-pattern nominalization of the piel כִּפֶּר, never a free-standing common noun, and it occurs only in bound phrases (יוֹם הַכִּפֻּרִים, Leviticus 23:27, 23:28, 25:9; חַטַּאת הַכִּפֻּרִים, Exodus 30:10 and Numbers 29:11; אֵיל הַכִּפֻּרִים, Numbers 5:8; כֶּסֶף הַכִּפֻּרִים, Exodus 30:16; with the prepositional עַל־הַכִּפֻּרִים of Exodus 29:36 standing slightly apart). Grammarians classify the plural either as a plural of abstraction (so Gesenius, Joüon-Muraoka, Waltke-O'Connor on the type זְקֻנִים, נְעוּרִים, רַחֲמִים) or as an intensive plural; Lunn (JNSL 42, 2016) argues abstraction fits nouns of this kind better, since no countable singular is in use. Baruch Schwartz explains the plural functionally: kipper covers all three actions of the day (purging the sacred area and its objects, the scapegoat rite, and the cleansing of the people), hence a "day of purifications".
On the underlying root, the nineteenth-century derivation from Arabic kafara "to cover" has largely been abandoned as the primary sense of the cultic verb, on three grounds: the verb's usage purges objects (altars, sancta), which "cover" does not describe; Barr's critique of the root fallacy (The Semantics of Biblical Language, 1961) discredited etymology-driven semantics; and the Akkadian kuppuru "wipe off, ritually purify" (CAD K, 178-179) gave "purge" a stronger comparative footing. Feder (VT 60, 2010) nonetheless denies a demonstrable direct derivation of כִּפֶּר from kuppuru and warns against conflating the concrete Akkadian term with the abstract Hebrew one; his Blood Expiation in Hittite and Biblical Ritual (2011) instead grounds the biblical blood rite in the Hurro-Hittite zurki tradition. Levine, following Landsberger, unifies "wipe off" and "smear on" under a common notion of rubbing. Genesis 6:14 כָּפַר "coat with pitch" is usually treated as a homonym.
The substantive debate concerns what kipper effects. Milgrom (Leviticus 1-16, Anchor Bible, 1991, esp. the kpr excursus at 1079-1084; and his 1976 Revue Biblique article comparing the sanctuary to the portrait of Dorian Gray) argues the חַטָּאת is a purification offering whose blood acts as a ritual detergent on the sanctuary and its sancta rather than on the offerer, with impurity penetrating on a graded scale and the adytum purged only on the annual day. Levine (In the Presence of the Lord, 1974; JPS Leviticus, 1989) roots kipper in כֹּפֶר and foregrounds ransom and substitution, building on Rashi's reading of blood as life. Gese and Janowski (Sühne als Heilsgeschehen, 1982) develop an inclusive-substitution model built on the hand-leaning gesture. Kiuchi (The Purification Offering in the Priestly Literature, 1987) argues against Milgrom that the offering purifies persons as well as sancta, and adds the priest's guilt-bearing role. Sklar (Sin, Impurity, Sacrifice, Atonement, 2005) offers the widely followed mediating position: sin and major impurity both endanger (calling for ransom) and pollute (calling for purgation), so kipper effects both, with the lifeblood doing both jobs. Gane (Cult and Character, 2005; JBL 127, 2008, 209-222) accepts much of Milgrom's framework but argues for a two-phase system and a privative מן in the kipper formula, so that ordinary purification offerings remove sin from the offerer. Gilders (Blood Ritual in the Hebrew Bible, 2004) urges interpretive restraint, treating blood manipulation as indexical and glossing kipper minimally as "effect removal"; Greenberg (A New Look at Atonement in Leviticus, 2019) argues instead for relational repair, and Feldman (The Story of Sacrifice, 2020) consistently renders kipper in P as "decontaminate".
A literary-historical question bears specifically on the noun. Knohl (The Sanctuary of Silence, 1995), followed by Milgrom, assigns Leviticus 23 and 25 to the Holiness School and Leviticus 16 to the Priestly Torah; Leviticus 16 describes the rite at length using only the verb (16:16, 16:20, 16:30, 16:34), never the noun phrase, so the fixed calendrical name looks like a later summary label. Frankel goes further, treating Leviticus 16:29-34a as an H-associated supplement that first fixes the rite to an annual date; Schwartz defends the literary unity of Leviticus 16. In the Septuagint the word group is rendered with the ἱλασ-/ἐξιλασ- family, יוֹם הַכִּפֻּרִים becoming ἡμέρα (τοῦ) ἐξιλασμοῦ (with ἱλασμός in some witnesses at Leviticus 25:9) and כַּפֹּרֶת becoming ἱλαστήριον; the Vulgate's dies expiationum preserves the plural. Mishnah Yoma preserves the plural name while developing a moral-theological account of kapparah, and the ἱλασ- group carries the reception into the New Testament (Romans 3:25; 1 John 2:2; Hebrews 9-10).