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WHATIS stands for 'World-wide Host-based Acronym Translation Information System.'

It used to be a database on the PROFS (OfficeVision/VM) system that all IBMers used before somebody forced them all to use Lotus Notes.

WHATIS is now Web-based rather than Host-based, but the acronym has not changed, unless you want to call it 'Web-hosted Acronym Translation Information System.'