What is the term called that describes the transmission of data in parts (via headers or body)?

The term describing the transfer of data in parts through headers or body is called chunked transfer encoding.

It is a mechanism in the HTTP/1.1 protocol that allows the server to send data to the client in chunks without knowing the total content size in advance. Each chunk contains its size and data, enabling efficient streaming or dynamically generated content transfer.

Example header:

Transfer-Encoding: chunked

This approach is useful for transmitting large or on-the-fly generated data without the need to buffer the entire content.