The term "cordless" should not be confused with the term "wireless", although it often is in common usage, possibly because some cordless devices (e.g., cordless telephones) are also wireless. The term "wireless" generally refers to devices that use some form of energy (e.g., radio waves, infrared, ultrasonic, etc.) to transfer information or commands over a distance without the use of wires and usually does not refer to the connection to the mains power supply (or lack thereof). The term "portable" is an even more general term and when used to refer to electrical and electronic devices this term generally refers to devices which are totally self contained (e.g., have built-in power supplies, have no base unit, etc.) and which may also use wireless technology.