Passage : Let $x=f\left(\theta \right)$  and $y=\varphi \left(\theta \right)$  . Now for all $\theta \in R$ , the point $\left(x,y\right)$  defines a locus. Thus the relation between x and y is indirectly conditioned by the variable $\theta$  . The variable $\theta$   is called the parameter

and $x=f\left(\theta \right),y=\varphi \left(\theta \right)$  are called the parametric equations of the locus.