The American Daylily Society definition: "A color pattern on the floral segments in which the base color and the veins within these tepals are of a contrasting color."
Daylilies show varied amounts of veining contrast with this color pattern. If you look carefully, some veining is quite subtle. I added some examples to clarify the many ways in which veining can appear.
Pictured below: Profound Mystery, Bordeaux Beauty, Mr. Lucky, Solar Eclipse, Pork Barrel Spending, and Waxed Legs.