THE Italian cook on the TV show was using a knife to cut up vegetables. The instrument she was using wasn't an ordinary knife, however, but a curved blade with handles at both ends. She had a hand on each handle and rocked the device back and forth to chop the food. ''This is my mezzaluna,'' she said, holding up the half-moon knife. That strange name reminded me that there are many things we see and use, sometimes frequently, without ever knowing their proper names. Things, for instance, like a ''zarf,'' a ''tang,'' or even an ''aglet.'' What are these mysterious things that have such odd-sounding names? Well, a ''zarf'' is a cup holder for hot drinks; a ''tang'' is the projecting prong on a tool or instrument, or the end of the knife blade that fits into the handle; and an ''aglet'' is the covering on the ends of shoelaces. Here are the names of some other everyday objects: Bibcock: a faucet with a bent-down nozzle. Bobeche: a disk placed around the top of a candle to catch the drippings. Duff: decaying vegetation on a forest's floor. Escutcheon: the plate around a keyhole or bathroom faucet. Hemidemisemiquaver: a 64th note in music. There is also a demisemiquaver and semiquaver. Phosphenes: the bright lights you see when you press your eyes shut tight. Philtrum: the small indentation below the nose and above the top lip of a human face. Quarrel: a small diamond-shaped pane of glass used in lattice windows. Zest: a thine piece of orange or lemon peel used for flavoring in a recipe.