nonstop
Describes an action, journey, or event that takes place without interruption or a break.
Wir sind von Hamburg nach München nonstop durchgefahren.
We drove nonstop from Hamburg to Munich.

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Describes an action, journey, or event that takes place without interruption or a break.
Wir sind von Hamburg nach München nonstop durchgefahren.
We drove nonstop from Hamburg to Munich.

'Nonstop' can be used as an adverb that refers to a verb. It can also appear as part of a compound noun such as 'Nonstop-Flug' (nonstop flight), often with a hyphen.
Die Kinder haben den ganzen Nachmittag nonstop gespielt.
The children played nonstop all afternoon.
'Nonstop' is a loanword from English. It is made up of 'non' (not) and 'stop' (halt/stop) and therefore means 'ohne Halt' (without a stop) or 'ohne Unterbrechung' (without interruption).
Das Festival bietet drei Tage lang nonstop Musik.
The festival offers nonstop music for three days.
The word is often combined with verbs of movement or continuous action. Typical examples are 'nonstop fliegen' (to fly nonstop), 'nonstop fahren' (to drive/travel nonstop), 'nonstop reden' (to talk nonstop), or 'nonstop arbeiten' (to work nonstop).
Er kann stundenlang über seine Projekte nonstop reden.
He can talk nonstop about his projects for hours.
direct
'direkt' and 'nonstop' can both express that something happens without interruption or detour. But 'direkt' fits especially when you take the shortest route or something happens immediately. 'nonstop' emphasizes that there is no break, for example during a trip, work, or an event. The words can be confused because both can mean 'without anything in between'. In practice: for a route or point in time, 'direkt' sounds more natural; for continuous movement or work without a break, 'nonstop' is more likely.

Die Mitarbeiterin hat vor der Messe drei Stunden nonstop telefoniert.
Before the trade fair, the employee spent three hours on the phone nonstop.
Nach dem Termin fahre ich direkt nach Hause.
After the appointment, I'm going straight home.