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A small chunk of data with a destination address — what your computer actually sends across the internet.
A small unit of data with a header containing source and destination addresses, plus a payload. Internet data is broken into packets (typically ~1500 bytes) so individual losses can be retransmitted without resending the whole stream.
An IP datagram. Encapsulates a transport-layer segment (TCP/UDP) and is routed independently across the network via best-effort delivery. Fragmentation occurs when a packet exceeds an interface MTU; reassembly is the receiver's job.