An angle bracket is the combination of a bra and ket (bra+ket = bracket) which represents the inner product of two functions or vectors (or 1-forms),
in a function space, or
in a vector space. Here, 
represents the adjoint.
The expression 
is also commonly used to denote the expectation
value of a variable 
.
See also
Bra, Brace, Bracket, Differential k-Form, Expectation Value, Ket, One-Form, Parenthesis, Square Bracket
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References
Bringhurst, R. The Elements of Typographic Style, 2nd ed. Point Roberts, WA: Hartley and Marks, p. 271, 1997.
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Cite this as:
Weisstein, Eric W. "Angle Bracket." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/AngleBracket.html