What is the meaning of Quadratic_form?

A homogeneous polynomial of degree 2 in a given number of variables.

A scalar quantity of the form ${\displaystyle \varepsilon ^{T}\Lambda \varepsilon }$, where ${\displaystyle \varepsilon }$ is a vector of n random variables, and ${\displaystyle \Lambda }$ is an n-dimensional symmetric matrix.

Source: wiktionary.org