A small piece of custom Hardware attachable to a computer to perform a narrowly defined task. Dongles do not contain general-purpose CPU, memory or I/O facilities.
In the 1980s and 1990s the term described RS232 or LPT attached appliances acting as CopyProtection for high-end applications. Dongles in this capacity acquired the image of being unwanted liabilities, not only because of their purpose, but also because of a whole slew of technical problems. They either took up a precious port or caused problems with other devices connected thru them. It was usually impossible to daisychain Dongles, should several applications require them. As if all that was not enough, their connectors would often start failing within short time if un/plugged too often. As a result, these days CopyProtection Dongles are a rare sight.
The term however is being recycled as for USB attached, highly specialized devices, often containing RandomNumberGenerators, secure key stores in cryptographic systems, or suchlike.
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