1) Payment system 
A counterparty (sometimes contraparty) is a legal entity, unincorporated entity or collection of entities to which an exposure to financial risk might exist. The word became widely used in the 1980s, particularly at the time of the Basel I in 1988. Sometimes it is used instead of unicorporation. The legal entity notion is using as counterparty. We...[technology] Counterparty is a peer-to-peer financial platform and distributed, open source Internet protocol built on top of the Bitcoin blockchain and network. Counterparty provides users with the world`s first functioning decentralized digital currency exchange, as well as the ability to create their own virtual assets, issue dividends, ...
The other participant, including intermediaries, in a swap or contract.
Party on the other side of a trade or transaction. 
The parties to an interest rate swap.
A person or company with whom one transacts. E.g. banks involved in a foreign exchange transaction with each other are counterparties.
The opposing side(s) of a transaction undertaken.