Hedging instrument is a designated derivative or (for a hedge of the risk of changes in foreign currency exchange rates only) a designated non-derivative financial asset or non-derivative financial liability whose fair value or cash flows are expected to offset changes in the fair value or cash flows of a designated hedged item (IAS 39 paragraphs 72–77 and AG94–AG97 elaborate on the definition of a hedging instrument).
Source: IAS 39 Financial Instruments: Recognition and Measurement.