The International Organization for Standardization. Is to standards organizations wordwide what the UN is to the national governments. See: http://www.iso.org for more details.
Sometimes this acronym also means In Search Of.
From an open source perspective, the ISO unfortunately is an old-economy standardization organisation, i.e., they won't let you read their publications unless you buy them. Standards effectively available from other sources includes
Category Glossary (although ISO works as an acronym in some languages, their website claims the name is really derived from the Greek word isos)