An iButton is a microchip similar to those used in a smart card but housed in a round stainless steel button of 17.35mm x 3.1mm - 5.89mm in size (depending on the function). The iButton was invented and is still manufactured exclusively by Dallas Semiconductor mainly for applications in harsh and demanding environments.
Like a smart card, an iButton does not have an internal power source. It requires connection to a reader (known as a Blue Dot Receptor) in order to be supplied with power and to receive input and send output. Unlike some smart cards, there are currently no contactless iButtons: they require physical contact with a reader to function.
There are iButtons that measure temperature (for food storage and transport); have an electronic ID (for physical access to buildings); and store e-cash (for purchases both in stores and via the web). For e-commerce applications, the iButton can support JavaCard 2.0/OpenCard standards in addition to proprietary software.
iButtons have an advantage over conventional smart cards in term of durability and longevity. The stainless steel casing gives iButton a far greater ability to survive in a range of temperatures -- all versions are functional from -40 C to +70 C -- and in a much harsher range of environments (such as exposure to salt water and long term exposure to physical impacts) than the plastic smart card. For e-commerce and personal ID usage, iButtons can be mounted on a range of personal accessories: watch, ring, key chain, or dog tag.
Among the major successes for the iButton have been its use in Turkey as an e-purse for the mass transit system; in Argentina and Brazil for parking meters; and in the United States as Blue Mailbox attachments that improve postal efficiency.