A bra 
is a vector living in a dual
vector space to that containing kets 
. Bras and kets are commonly encountered in quantum
mechanics. Bras and kets can be considered as 1-vectors and 1-forms (or vice versa),
although this is almost always done only in a finite-dimensional vector
space.
Considered as an inner product, the bra and ket form an angle bracket (bra+ket = bracket) 
.
See also
Angle Bracket, Covariant Vector, Differential k-Form, Ket, L2-Inner Product, One-Form
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References
Dirac, P. A. M. "Bra and Ket Vectors." ยง6 in Principles of Quantum Mechanics, 4th ed. Oxford, England: Oxford University Press, pp. 18-22, 1982.
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Cite this as:
Weisstein, Eric W. "Bra." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/Bra.html