The name given to the trail of afterburn (of a jet perhaps?)

What is the name of the trail (of after burn) left by a jet?

I know it's something nearly like "after burn", but I can't be sure as I've not used it for quite a long time!

Contrails from a four-engine fixed wing aircraft

If you are referring to the cirrus-cloud-like vapor trails left by jets, the name for those is contrails (short for condensation trails), vapor trails, or even cirrus aviaticus.

Edited to add: Chemtrails is an 'allegation' more than a fact. I include it here because it is still unclear what OP wants a name for. Since he included after burn, I thought he might be referring to fuel residue. While I think contrails is what he's after, I also recognize that fuel does not burn with 100% efficiency, so there actually exist pollutants in (and around) contrails; I have no idea how quickly the pollutants diffuse out of the moisture-laden contrail. But I am no believer in chemtrails as the term is used by… other folks.

Also wingtip vortices are usually invisible; they may become visible under certain unusual conditions unlikely to be encountered from large aircraft, so it is unlikely OP is looking for that as an answer.

If you are specifically referring to an afterburner (where fuel is injected directly into the jet exhaust to generate extra thrust) there is a characteristic pattern of flames fronts produced called shock diamonds ( or sometimes dancing diamonds )

enter image description here

You might be referring to jet wash, which the heated air coming out of a jet engine when the jet accelerates on take off.

Here is a video (Temora sabre take off with jet wash, crazy) of that happening, taken by what appear to be group of British aviation enthusiasts. I mention this because you specifically tagged this for British English. (Starts at the 1:00 mark, if you want to jumpy right to it.

Here is another example, with the caption "Look at that jet wash" enter image description here