Learn Polish: Wyjazd jest o ósmej. - The departure is at eight o'clock.

Wyjazd is the noun form of the perfective verb wyjechać (to depart or drive out). Just like the verb, it combines the prefix wy- (out) with the root for riding or driving (-jazd-, related to jechać). So wyjazd specifically means a departure by a vehicle, or a trip.

Why do we say o ósmej instead of just o osiem?

To say "at a specific time" in Polish, you use the preposition o followed by an ordinal number (first, second, eighth) in the Locative case. Ósmej means "eighth". We use the feminine ending because it has to agree with the word godzinie (hour), which is usually left out in this phrase. You are literally saying "at the eighth hour".

If wyjazd means a departure by vehicle, what word would I use if the departure is on foot?

If the departure is on foot, you would use wyjście (an exit or a departure on foot), which comes from the verb wyjść (to walk out). Wyjazd is strictly for when a vehicle is involved, like a car, train, or bus.