lightning vs. lightening

a brilliant electric spark discharge in the atmosphere, occurring within a thundercloud, between clouds, or between a cloud and the ground.

to emit a flash or flashes of lightning (often used impersonally with it as subject):

of, relating to, or resembling lightning, especially in regard to speed of movement: lightning speed.

the descent of the uterus into the pelvic cavity, occurring toward the end of pregnancy, changing the contour of the abdomen and facilitating breathing by lessening pressure under the diaphragm.