Notation
If a norm p: V → R is given on a vector space V then the norm of a vector v ∈ V is usually denoted by enclosing it within double vertical lines: ‖v‖ := p(v). Such notation is also sometimes used if p is only a seminorm.
For the length of a vector in Euclidean space (which is an example of a norm, as explained below), the notation |v| with single vertical lines is also widespread.
In Unicode, the codepoint of the "double vertical line" character ∥ is U+2016. The double vertical line should not be confused with the "parallel to" symbol, Unicode U+2225 ( ∥ ). This is usually not a problem because the former is used in parenthesis-like fashion, whereas the latter is used as an infix operator. The single vertical line | is called "vertical line" in Unicode and its codepoint is U+007C.
Read more about this topic: Norm (mathematics)