In category theory and its applications to other branches of mathematics, kernels are a generalization of the kernels of group homomorphisms and the kernels of module homomorphisms and certain other kernels from algebra. Intuitively, the kernel of the morphism f : X → Y is the "most general" morphism k : K → X that yields zero when composed with (followed by) f.
Note that kernel pairs and difference kernels (aka binary equalisers) sometimes go by the name "kernel"; while related, these aren't quite the same thing and are not discussed in this article.
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Definition
Let C be a category. In order to define a kernel in the general categorytheoretical sense, C needs to have zero morphisms. In that case, if f : X → Y is an arbitrary morphism in C, then a kernel of f is an equaliser of f and the zero morphism from X to Y. In symbols:
To be more explicit, the following universal property can be used. A kernel of f is any morphism k : K → X such that:
 f o k is the zero morphism from K to Y;
 Given any morphism k′ : K′ → X such that f o k′ is the zero morphism, there is a unique morphism u : K′ → K such that k o u = k'.
Note that in many concrete contexts, one would refer to the object K as the "kernel", rather than the morphism k. In those situations, K would be a subset of X, and that would be sufficient to reconstruct k as an inclusion map; in the nonconcrete case, in contrast, we need the morphism k to describe how K is to be interpreted as a subobject of X. In any case, one can show that k is always a monomorphism (in the categorical sense of the word). One may prefer to think of the kernel as the pair (K,k) rather than as simply K or k alone.
Not every morphism needs to have a kernel, but if it does, then all its kernels are isomorphic in a strong sense: if k : K → X and l : L → X are kernels of f : X → Y, then there exists a unique isomorphism φ : K → L such that l o φ = k.
Examples
Kernels are familiar in many categories from abstract algebra, such as the category of groups or the category of (left) modules over a fixed ring (including vector spaces over a fixed field). To be explicit, if f : X → Y is a homomorphism in one of these categories, and K is its kernel in the usual algebraic sense, then K is a subalgebra of X and the inclusion homomorphism from K to X is a kernel in the categorical sense.
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