At its most abstract an Instrument is a category of trade types

Detail

It is difficult to define Instrument further than this, because the term is used differently between organizations, functions within organizations, and trading systems

A typical instrument may have dimensional attributes of:

• Trade type
• Commodity
• Settlement type (physical or financial)
• Maybe tenor (day, week, month etc.)
• Maybe market/location

So, examples of Instruments may be

• Physical Forward 
• UK Power Swap
• API#4 Coal Futures

Note the similarity and differences to a Product

Note that some proprietary systems make a very specific use of the term Instrument