Sometimes it's used to mean "defined as". I.e. you're saying "Let <LHS> be the value computed by <RHS>", rather than saying "<LHS>, which is defined elsewhere,

Sometimes it's used to mean "defined as". I.e. you're saying "Let <LHS> be the value computed by <RHS>", rather than saying "<LHS>, which is defined elsewhere, is equal in value to <RHS>". Other times it's used to indicate equivalence or congruence with respect to some equivalency class. In other words, it's a weaker form of equality, the opposite of how's it's used in, say, Javascript. For instance, (1, 2) === (2, 4) over the equivalency class of the rational numbers. The latter is how it's used in this context. a === b (mod c) means that "a is congruent to b under the mod c equivalency relation".