A pullback of two morphisms 
and 
in a category is an object 
with morphisms 
and 
satisfying 
and the following universal
property. For every object 
with morphisms 
and 
satisfying 
, there is a unique morphism 
such that 
and 
, where 
denotes composition
of morphisms.
For schemes, the pullback is the fiber product 
.
In any category admitting the pullback of 
, the object is often denoted 
and called a fiber product.
