Calipers is the American spelling of callipers. The common meaning is a tool with two legs joined at the top, used to measure how wide something is — a pipe, a piece of wood, even a person’s waist in an old doctor’s room. You open the legs, fit them to the object, then read the gap. There is also a medical sense: metal braces worn on a weak or injured leg to help a person walk. That sense is more British in flavour, but American texts may still spell it calipers. In everyday US English the measuring tool is the meaning you will meet first. The word is usually plural in form, like scissors: these calipers. Though actually the name is a changed, slightly broken form of caliber, the “width” word — the tool took the name of the thing it measures.