The closest term I can think of or find is "correlated", but that has a bunch of different possible interpretations.
Over the interval $(0, \infty]$, the two variables of each of these functions satisfy the relation that I'm talking about:
The blue and green lines ($x$, $kx^3$) have pairs of variables that satisfy it over their whole domain.
I also imagine prepending "negatively" to the word just as you can say "negatively correlated" to specify that if one goes up, the other goes down, and vice versa.