It's one common notation for stating that the left-hand side is defined as (in contrast to equal to) the expression on the right-hand side.
In mathematical notation, what are the usage differences between the various approximately-equal signs "≈", "≃", and "≅"? The Unicode standard lists all of them inside the Mathematical Operators Block.
notation - Difference between "≈", "≃", and "≅" - Mathematics Stack ...
A better, but less common, notation is $\varepsilon$ (formatted with $\varepsilon$). You can find answers and comments related to that latter symbol $\varepsilon$ in the linked posts.
notation - Symbol for “such that” (not in set) - Mathematics Stack Exchange
The use of natural language is often more effective when presenting an idea. In my opinion the less often a symbol is used where a few words can go, the better. That said, there are many places where symbols are useful and simplify matters. The word "where" can often be replaced with "such that", and corresponding to this we have a few regularly used symbols. For instance, in set builder ...
$\mathbb Q$ is used to represent rational numbers. $\mathbb R$ is used to represent reals. Is there a symbol or convention that represents irrationals. Possibly $\mathbb R - \mathbb Q$?
Notation for definition and equivalence Ask Question Asked 12 years, 8 months ago Modified 3 years, 1 month ago
The answer (as is often the case) come from Latin. "versus" simply means against and is used in the sporting context as well. We say that in some contest "Team A versus team B", meaning team A is against team B. The graph is the same - one variable is plotted against (or versus) another. From the same cognate root we also get the English "adversary".