It's been mentioned a few times, but not really explained in lectures.
The splanchnic organs are the viscera i.e. the organs occupying the abdomen e.g. liver, intestines, pancreas, kidney, so the term "splanchnic nerves" is simply a collective term for those nerves that supply these visceral organs, which are spinal roots of the appropriate region of the spinal cord; in the case of the lumbosacral nerve roots, they will carry mostly sympathetic fibres (sympathetic division of autonomic efferent outflow), but sensory fibres may also be present.
Each member of the "splanchnic nerve" group will have its own specific name, depending partly on its distribution.