The story
Concatenation of the terms bra and ket, which were introduced in 1939 and in turn derive from the splitting of the English word bracket.
Etymology from Wiktionary, CC BY-SA 4.0.
bra-ket
Today it means: 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). Full definition →