Note that while there is a singular form, nopal, it refers to the plant only -- the pads properly are always treated as a plural noun. The definition above elides over the, ah, prickly issue of which species of prickly pear are edible (which is just as well, as the exact divisions of that unruly genus is far from settled) but the farmed plants are usually Opuntia ficus-indica (Indian fig) or Opuntia joconostle. Adopted in the 1570s (yes, that long ago!) from Mexican Spanish, from nopal, prickly pear, from Nahuatl nōpalli.
Both nopales and tunas (prickly pear fruits) have gained popularity as a healthy foods.