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"Viajar" is an intransitive verb which is often translated as "to travel", and "de viaje" is a phrase which is often translated as "on a trip". Learn more about the difference between "viajar" and "de viaje" below.
viajar(
byah
-
hahr
)An intransitive verb is one that does not require a direct object (e.g., The man sneezed.).
a. to run 
El metro viaja hasta las dos de la mañana los sábados.The metro runs until two in the morning on Saturdays.
de viaje( 
deh
byah
-
heh
)A phrase is a group of words commonly used together (e.g., once upon a time).
a. on a trip 
Nos pillas de casualidad. Estamos a punto de salir de viaje para Roma.You're lucky to catch us. We're just about to leave on a trip to Rome.
b. away on a trip 
El señor López está de viaje y no vuelve hasta mañana. ¿Quiere dejarle algún recado?Mr. Lopez is away on a trip and won't be back until tomorrow. Do you want to leave a message for him?
c. away 
Mi padre trabajaba para una multinacional y estaba siempre de viaje.My father used to work for a multinational company, and he was always away.
d.  no direct translation
This refers to an idiomatic word or phrase for which there is no word-for-word translation.
Ya llevan un mes de viaje y se proponen dar la vuelta al mundo.They've already been traveling for a month and are planning to go around the world.
Todavía nos quedan cinco horas de viaje por delante hasta llegar a Hong Kong.We still have a five-hour trip ahead of us before we arrive in Hong Kong.
a. travel 
Los gastos de viaje de nuestros representantes corren a cargo de la empresa.Our representatives' travel expenses will be covered by the company.