The rump kernel can be used as the basis of a unikernel (see: Rumprun). But, the rump kernel theoretically has applications beyond just use in a unikernel, by providing a common, documented API on one end and hypercall interface on the other which (in theory) enables robust driver and application portability even when used in "traditional" userland / full-OS environment.

So, in short, a rump kernel is a building block that can be used to build a unikernel, but the rump kernel can also be used in other contexts as well.