A broad range of securities and contracts that are traded in the financial markets, including transferable debt and equity securities, money-market instruments, units in collective investment undertakings, options, futures, swaps, forward rate agreements and other derivative contracts, and emissions allowances.
A detailed definition, cross-referred to in many other EU measures, is set out in the Markets in Financial Instruments Directive 2014/65/EU (MiFID II) Annex 1, Section C.