How you call the relationship between variables $X$ and $Y$ if $X=1-Y$

Question

How you call the relationship between two variables where one is equal to 1 minus the other. For example, if my variables are $X$ and $Y$ and I have that $X=1-Y$

I what to say the $X$ is the ________ of $Y$.

Like I would say $X$ is the reciprocal of $Z$ if $X=\frac{1}{Z}$

Answer 1

In probability, the event $A^c$ (with probability $1 - P(A)$) would usually be called the complement of A.

In set theory, we have the complement of the set $A$, $A^c$, which is equal to $\Omega - A$ if $\Omega$ is the ‘universe’ set.

We also have the notion of the radix complement of an $n$ digit number $x$ in base $b$, which is $b^n - x$.

I think calling $X$ the complement of $Y$ is a good choice here.

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Another view: the points satisfying the equation $X+Y=1$ form a line in the plane, so perhaps the word linear is also an option.