Definition: A function of the form
\[
y(x)=ax^2+bx+c,
\]
where $a$, $b$ and $c$ are numbers ($a\neq 0$) is called a quadratic function.
Remark: For $a=0$ this function is either
linear (if $b\neq 0$) or
constant (if $b=0$).
Example: $g(x)=4x^2+8x+3$ is an example of a quadratic function.
