I am not sure if there is any Standards based definition,
Vernacular though would be something like "a pipe feeding other pipes, or a mainline feeding sub-systems".

We would use that term to describe a 6" pipe feeding 10 2" pipes feeding smaller sub-systems.

A pump station with three pumps might have a common intake "header", with 3 smaller lines feeding fluid to each of the three pumps.

A hydraulic system might have a 1" supply "header" with 20 1/4" lines being fed off that 1" supply header.

PUMPDESIGNER

A header is a large pipe or manifold that is used to collect the discharge from many smaller pipes( a collecting header) , or can be used to distribute fluid to many smaller pipes ( distributing header).

A header can also be used for purposes of pressure equalization ( a partial mix header) and also to mix fluids to provide equal outlet enthalpy ( full mix header)

The term 'header' can mean a number of things depending in what context you use it.

If you are installing water to serve 5 wash basins, then you could install a 1" pipe to feed the first basin, reducing down to 3/4" to the next basin and so on. This is messy and so you could instead install a single 'header' pipe with smaller branch connections to serve each basin. This looks neater and is easier to fit.

Alternatively you may wish to join 3 or 4 boilers together using a large 'header' pipe. This could be a reverse return header or a 'low-loss' header depending on your installation/design approach.

Hope this helps

Drapes

As for this forum, header is a pipe, conduit, or chamber which distributes or collects fluid from a series of smaller pipes or conduits; an example is a manifold.

Other disciplines such as communications, building and civil, electrical, mechanical, and mineral engineering have different definitions for the same word.

Thank you very much!

Your answers really helped me!