Split algebraic group

Definition
An algebraic group over a field $$K$$ (not necessarily algebraically closed) is termed a split algebraic group if its Borel subgroup has a composition series such that all the composition factors (i.e., all the successive group quotients) are isomorphic to either the additive group of $$K$$ or the multiplicative group of $$K$$.

Note that if $$K$$ is an algebraically closed field, then every linear algebraic group (and hence, every affine algebraic group) over $$K$$ is split.