Where does the "inertia" term come from?

Question

Where does the "inertia" term in regard to quadratic forms (e.g. Sylvester's law of inertia) come from?

Thank you!

Answer 1

Inertia in latin means not changing, inert. If you reduce a quadratic form to a canonic form, the number of terms at positive coefficient and the number of terms at negative coefficients don't change changing the canonic form.