Symbols:K

Subgroup

 * $K$

Frequently used, in conjunction with $H$, to represent a general subgroup of a given group $G$ when two such subgroups need to be discussed.

Field

 * $K$

One of the letters used as a variable denoting a general field.

Field as Algebraic Structure

 * $\struct {K, +, \circ}$

The full specification for a field, where $+$ and $\circ$ are respectively the addition and product operations.

Kernel

 * $K$

Often used to denote the kernel of a general homomorphism