The terms can be TCP/IP layer dependant. For example, rfc 1122 says a datagram is a "connectionless protocol" if I'm correct. Fine. UDP is at transport layer, IP and ICMP is at Internet layer. IP is called datagram. This can get confusing. Of course there is potential in defining layer dependant terms of getting confusing as well. Terms like packet and datagram can become more inclusive, in an official way to be layer dependant according to TCP/IP.