Definition of Network Units: Fragment, Segment, Packet, Frame, Datagram

The answer above is complete. Nevertheless, I think the term packet came from packet switching if I remembered that correctly ..

There are 2 types :

Circuit switching is a methodology of implementing a telecommunications network in which two network nodes establish a dedicated communications channel (circuit) through the network before the nodes may communicate. The circuit guarantees the full bandwidth of the channel and remains connected for the duration of the communication session. The circuit functions as if the nodes were physically connected as with an electrical circuit.(Wiki)

Packet switching is a digital networking communications method that groups all transmitted data – regardless of content, type, or structure – into suitably sized blocks, called packets.(Wiki)

I would have defined it myself, but just to be sure.

So here is the Packet Term. In regard of fragment I concur with Josua Schmid's answer.

Packet: The basic unit of communication between a source and a destination in a network is a packet. Packets are data units within the network layer in the OSI model. The packet is in a Network layer.

Fragment:Packets can often be larger than the maximum size, so each packet is also divided into smaller pieces of data called fragments. The Network layer is responsible for fragmentation.

Segment: A segment is a broken piece of a packet with a TCP header in each of them. segments increase the efficiency of network performance and improve security. Segments are in a Transport Layer.

Frame:Similar to a Packet, frames are small parts of a message in the network. a frame is the unit of data used in the OSI model’s data link layer.

Datagram:The datagram represents a data unit of transfer in networking. Data transmitted in a network is divided into smaller parts called datagrams. Transport layer

In addition, there is something class bit, this is in a Physical layer