Payload
In plain terms
The actual data carried in a request or response.
Going deeper
The payload is the meaningful content of a message, separate from headers and metadata. It's what the request or response is really delivering.
Under the hood
Often JSON; size affects performance, and validation/sanitization of payloads is a security concern.
Examples
JSON request body, API response data, message body, POST data.
Think of it like...
Like the contents of a package, as opposed to the shipping label and box.