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 
compatible with the four specified morphisms.
For schemes, the pullback is the fiber product 
.
Pullbacks are also called fiber products in other categories.
