TWAIN is a standard that was launched in 1992 by members of the industry that realised there was a need for a standard protocol and API to communicate with imaging devices, such as scanners. The standard comprises of three parts, the application, the source manager software and the data source software.
The word TWAIN is not an acronym. It comes from a quote from Rudyard Kipling's "The Ballad of East and West" - "...and never the twain shall meet...". It is meant to symbolise the problems of connecting scanners and PC's at the time. The upper case was used to make it more distinctive and this gives it the appearance of an acronym.
A contest was held to find a potential meaning for the acronym but eventually, none were used. "Technology Without An Interesting Name" was one of the entries and has been adopted by popular culture.