Meaning – The term peer, refers to any functional unit that is in the same layer as another entity. For networking and distributed systems; a lot of it is either “client-server” (HTTP, FTP, NTFS) or “hierarchical” (NTP, NNTP, DNS); where a client and server are not peers because they aren’t equal, and where clients communicating with the same server aren’t necessarily peers because there may not be a relationship between the clients.
However, from a different perspective (at the physical layer) it’s extremely likely that the hosts are considered equals (regardless of which higher-level protocols are being used) and from that perspective, the hosts would be considered peers.
Example of usage – “Two desktop PCs in a network are peers. A desktop PC and a server are not peers as they perform different operations. The desktop PC may query the server for business data, but the server does not query the PC for the same data.”