What instrument is used to vary electrical resistance in a device?

Virtually all electrical devices (actually, all except for superconducting devices) have resistance. There is one type of electrical device that is found in almost every electronic circuit ever invented. It is used wherever the designer needs a specific amount of resistance in the circuit. I'd hate to spoil your fun by simply telling you its name. Think of all of the different names of electrical devices that your teacher has told you about. (or look in the index of your text book if you're in a hurry) Is there one that comes to mind when you think of RESISTance?