Packet

Packet is a generic term for a unit of data sent across a network. It is the essential unit of transfer in the packet-switching transfer via TCP/IP.

W3E References:

datagram

A datagram is a single packet of information capable of being routing from source to destination.

ATM

Packets within asynchronous transfer mode are called cells.

Detail:

In a packet-switching model, data is divided into individual packets. Each packet has a header containing the information necessary to successfully route the packet to its destination. Packets on the Internet contain on average about 200 bytes.