A small piece of custom hardware attached to a computer for some special purpose.
Original the domain of copy-protection features for high-end applications Dongles have fallen from favour for copy-protection, but are re-inventing themselves as RandomNumberGenerators, secure key stores in cryptographic systems and so forth.
If a device contains general-purpose functionality, such as a general-purpose CPU, general-purpose memory or some IO facitilities then it's not a dongle, since the defining characteristic of the dongle is it's specialisation.
in the 1980s and 1990s, dongles had serial interfaces, but most modern dongles are USB Dongles.
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