Lightening is a non-clinical term that describes the time when the baby descends into the pelvic cavity in preparation for birth. In clinical terms this is referred to as engagement.
Linea nigra (Latin for "black line") is a dark vertical line that appears on the abdomen in about three quarters of all pregnancies. The brownish streak is usually about a centimeter in width. The line runs vertically along the midline of the abdomen from the pubis to the umbilicus, but can also run from the pubis to the top of the abdomen.