Encapsulation and PDU
It’s generally accepted (especially in Cisco/CCNA courses) that the PDU (protocol data unit) of IP, which maps to the Network layer in the ISO OSI reference model, is called “packet,” while UDP “datagram” and TCP “segment” at the transport layer as the following diagram shows:
IP Datagrams?
However, it’s not uncommon for people to come across the term IP “datagram.” Is it correct? Yes, absolutely.
The internet protocol provides for transmitting blocks of data called datagrams from sources to destinations, where sources and destinations are hosts identified by fixed length addresses.
Source: RFC 791
IP Datagrams and Fragmentation
The raw IP data units are called “datagrams,” which may be split into packets (IP fragmentation) to fit the maximum transmission unit (MTU) of underlying network technologies.
IPv4 allows fragmentation which divides the datagram into pieces, each small enough to accommodate a specified MTU limitation. This fragmentation process takes place at the internet layer. The fragmented packets are marked so that the IP layer of the destination host knows it should reassemble the packets into the original datagram.
Source: Wikipedia
