Yes, there is.
Two variable are in a proportional relationship if one is a multiple of other, i.e. if their ratio is constant.
In fact, two equivalent ways of saying that x and y are in a proportional relationship are saying that there exists a number k such that
[tex] y = kx\quad\text{or}\quad \dfrac{y}{x}=k [/tex]
In this case, you can see that the perimeter is always three times the side, which makes sense, because the perimeter of an equilateral triangle is composed of three "copies" of the side of the triangle.
So, the perimeter is a multiple of the side (with k=3), and in fact you have
[tex] p = 3s \iff \dfrac{p}{s} = 3 [/tex]
which are the two equivalent ways of saying that p and s are in a proportional relationship.
