A parabola is a type of conic section. It can be described algebraically by an equation of the form \(y=ax^2+bx+c\) (\(a\), \(b\), \(c\) constants, \(a\) nonzero).
The path of a thrown object is a parabola.
A parabola has an axis (the line of symmetry of the curve) and a focus on the axis. For the parabola with equation \(y=x^2/4a\), its axis is \(x=0\) and its focus is at \((0,a)\).
Light travelling towards a parabolic mirror parallel to its axis will be reflected to pass through the focus.