Notation for "Defined as proportional to" If I say $x$ is defined as $y+z$ then I can say $x := y+z$. If I want to say $x$ is defined as proportional to $y+z$, then how can I say that? Would I say $x :\propto y+z$?

The symbol $\propto$ is indeed meaning that. It means that there exists $k$ a constant in a $\mathbb{K}$ field so that $x=k\cdot (y+z)$.
