What is `|| < 3, 4 , 0 > ||`?

Related questions

• What are vectors used for?
• Why vectors cannot be added algebraically?
• How do we represent the magnitude of a vector in physics?
• How do you find the equation of a vector orthogonal to a plane?
• Why are vectors important?
• How does a vector quantity differ from a scalar quantity?
• How can I calculate the magnitude of vectors?
• How do vectors subtract graphically?
• How do force vectors affect an object in motion?
• How can vectors be represented?