Sometimes I see people say the phrase "client" when referring to the node. Why is it called that? Really its not a client, because that depends on the client-server model.
It's called client because it is a client that implements the network protocol to interface to the peer-to-peer network, rather than a client connecting to a server in the client-server model.
Functionality-wise, the p2p client will have a client-server "server" listening out for connections from other peer's client-server "client" connection requests, and the p2p client will also have a client-server "client" reaching out to connect to other p2p node's client-server "servers".
It is like a BitTorrent client - from Comparison of BitTorrent Clients:
The BitTorrent protocol coordinates segmented file transfer among peers connected in a swarm. A BitTorrent client enables a user to exchange data as a peer in one or more swarms. Because BitTorrent is a peer-to-peer communications protocol that doesn't need a server, the BitTorrent definition of client differs from the conventional meaning expressed in the client–server model.
And from BitTorrent:
To send or receive files one uses a BitTorrent client; a computer program that implements the BitTorrent protocol. Such clients include μTorrent, Xunlei, Transmission, qBittorrent, Vuze, Deluge, and BitComet. BitTorrent trackers provide a list of files available for transfer, and allow the client to find peers known as seeds who may transfer the files.