A pearl is a white, round gem formed inside an oyster or clam. Pearl can also refer to something that resembles the gem in color or shape, like a pearl of sweat that forms on your forehead when you're hot.
Why is a pearl called a pearl? It isn't clear where the Medieval Latin word perla comes from. Some believe it refers to the shape of the shell in which pearls are found: either pear, like the shapely fruit, or perna, literally "ham," because the shell is shaped like a pig's leg.