relation

A (binary) relation can be abstracted in mathematical terms as a set of ordered pairs, whose first component is related to the second component in a certain way. In general, a relation can be thought of as a subset of the cross product of two sets $A$ and $B$. Note that the two sets need not be disjoint.

A relation can also be defined using set builder notation like so: $B=\{⟨p,d⟩:\text{person p was born on date d}\}$

A more mathematically relevant example is the unit circle, which can be represented by $\{⟨x,y⟩:x^2+y^2=1\}\subseteq \mathbb{R}\times \mathbb{R}$.