  It is shown that universal algebras that are injective in their equational
classes are characterized by internal property that can be called completeness.
We define universal algebra $A$ as complete (closed to simple extensions) if
for each its subalgebra $A'$ and each set of extension conditions for this
subalgebra there is $a \in A$ that satisfies these conditions. We define a set
of extension conditions for $A'$ to some extension $A''$ as the difference
between factorization kernels of free algebras for $A''$ and $A'$. It's proved
that each injective universal algebra is complete and each complete universal
algebra belonging to the class of algebras with CEP is injective. It's checked
directly that complete (in the sense of ordering) boolean algebras and
divisible Abelian groups are complete in the sense defined here.

Commutative Algebra