For options, the security that is subject to purchase or sold upon exercise of an option contract. For example, IBM stock is the underlying security for IBM options. For depositary receipts, the class, series, and number of the foreign shares represented by the depositary receipt. Bloomberg Financial Dictionary
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The security that is affected by a corporate action or a meeting. Euroclear Clearing and Settlement glossary
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The instrument upon which traded options are listed. Exchange Handbook Glossary
Financial and business terms. 2012.