The bond, deliverable against a futures contract, for which delivery is most attractive in terms of cost from the short position holder`s point of view. 
The security or commodity in the cash market that is the cheapest possible means of delivering on a... <a target=_blank href='http://www.finance-glossary.com/terms/cheapest-to-deliver.htm?id=12649&ginPtrCode=00000&PopupMode=false' title='Read full definition of cheapest to deliver'>more</a>
The cash security that provides the lowest cost (largest profit) to the arbitrage trader. The futures price tracks the CTD instrument.