The "actual data" in a packet or file minus all headers attached for transport and minus all descriptive meta-data. In a network packet, headers are appended to the payload for transport and then discarded at their destination. In a key-length-value structure, the key and length are descriptive data about the value (the payload). See protocol stack
In the analysis of malicious software such as worms, viruses and Trojans, it refers to the software's harmful results. Examples of payloads include data destruction, messages with insulting text or spurious email messages sent to a large number of people.