1. A packet which includes both the source and destination addresses provided by the user, and not the network. Datagrams can also include data. From Linux Guide @FirstLinux

2. In protocols, a datagram is a single transmission that stands by itself. They are often known as unreliable datagrams because there is not guarantee that they will reach their destination. It is up to some higher protocol or application to verify that a datagram reaches its destination. Streaming media (audio/video/voice) often use datagrams because it doesn't really matter if a few are lost in transmission. From Hacking-Lexicon