datagram
One self-contained message sent on its own, like a postcard.
A datagram is a self-contained message delivered (or lost) as a unit, with no relationship to the datagrams before or after it.
Examples
A 17-byte JSON reading sent by UDP becomes a 25-byte datagram: 8 bytes of header plus the payload.
The laptop's receive call returns the whole 17-byte reading at once, because each datagram arrives whole or not at all.
See also