noun
contrail; vapor trail
The white trail left behind by an aircraft in the sky.
空に長い飛行機雲ができていた。
A long contrail had formed in the sky.
飛行機雲がしばらく消えなかった。
The contrail didn't disappear for a while.
Compound of 飛行機 (hikōki, 'airplane') + 雲 (kumo, 'cloud'). The kumo changes to gumo as an instance of rendaku (連濁).