For a linear map T:V→W between vector spaces, kerT, the kernel of T, is the subspace {v ε V | Tv = 0W} of V. It is also called the null space, and its dimension is called the nullity of T. See rank-nullity theorem.
For a homomorphism f:G→H between groups, the kernel is kerf = {g ε G | f(g) = eH}. It is a normal subgroup of G.
For a homomorphism f:R→S between rings (or modules), the kernel is kerf = {r ε R | f(r) = 0S}. It is an ideal (or submodule) of R.
In each of the above cases, a version of the first isomorphism theorem applies.