A category is abelian if it is preadditive and
- it has a zero object,
- it has all binary biproducts,
- it has all kernels and cokernels, and
- all monomorphisms and epimorphisms are normal.
Now we introduce the following concept: A category is left-abelian if it is preadditive and
- it has a zero object,
- it has all binary biproducts,
- it has all kernels and all cokernels of kernals, and:
- Let $f:X\rightarrow Y$ be a morphism in the category. It must factor (uniquely) through its cokernal of kernal $g:X\rightarrow Z$ by the universal properties. Hence we find a unique $h: Y \rightarrow Z$ such that $f = hg$. Now we demand that $h$ is a monomorphism.
Dually one can define the concept of a right-abelian category. One can show that a category is abelian iff it is left- and right-abelian. Are left- or right-abelian categories studied in the literature?