Bra-ket

noun

noun ·Rare ·Advanced level

Definitions

Noun
  1. 1
    The scalar product of two vectors (and optionally an operator), in Hilbert space, where the left (row) vector is a bra and the right (column) vector is a ket. Symbolised by ⟨...|...⟩ (bra and ket only) or ⟨...|...|...⟩ (bra, operator, ket).

Etymology

Concatenation of the terms bra and ket, which were introduced in 1939 and in turn derive from the splitting of the English word bracket.

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Data sourced from Wiktionary, WordNet, CMU, and other open linguistic databases. Updated March 2026.