Universal Set
The universal set is the set that contains all objects under consideration in a given context, often written as U.
Definition
The universal set $U$ is the "world" for a problem, containing all possible elements you are considering, and every other set in the discussion is a subset of $U$. In a given context, all sets satisfy $A \subseteq U$, and the complement $A' = U \setminus A$ depends entirely on what $U$ is; different choices of $U$ give different complements. In naive set theory, a "set of all sets" (universal set $V$) leads to Russell's paradox, so ZFC avoids this by making $V$ a proper class, not a set; in category theory, a Grothendieck universe is a set large enough to contain all the mathematical objects of interest, providing a safe context for working with "all" sets of a given size.
Example
If you are sorting students by grade level, $U$ might be "all students in the school," and every subset (like "9th graders") lives inside $U$. If $U = $ integers and $A = $ even integers, $A' = $ odd integers; but if $U = $ real numbers and $A = $ rational numbers, $A' = $ irrational numbers, same $A$, different $U$, completely different complement. In NBG (von Neumann-Bernays-Godel) set theory, proper classes like $V$ (all sets) and $\text{Ord}$ (all ordinals) exist but cannot themselves be elements of sets, resolving the paradox.
Key Insight
The universal set defines the boundaries of the conversation; changing $U$ changes what the complement of any set looks like, so always state or identify $U$ clearly before working with complements, a common source of errors in set theory problems. The impossibility of a universal set in ZFC is not a bug but a feature: it forces precision about the scope of mathematical arguments and motivates the study of large cardinal axioms.