Traveling is any collective party movement outside or in dungeons.
The DMG breaks travel down in increments ranging from 10ft to 6mi (DMG 242), and makes specific reference to traveling inside dungeons. This means that "traveling" can be very short or very long distances.
If you want to be super precise, DMG 242 further states that
The rules for travel pace in the Player's Handbook assume that a group of travelers adopts a pace that, over time, is unaffected by the individual members' walking speeds.
And PHB 181 only draws a distinction between ordinary movement and combat movement
Every character and monster has a speed, which is the distance in feet that the character or monster can walk in 1 round. This number assumes short bursts of energetic movement in the midst of a life- threatening situation. The following rules determine how far a character or monster can move in a minute, an hour, or a day.
This means that essentially any party movement that's not a combat can be treated as traveling.
In the case of the Revised Ranger's ability, this means that the benefit only goes away when the party stops moving, whether that's for combat or for resting, as long as the entire traveling period is at least one hour. The specific wording of the ranger ability makes references to traveling in the wilderness, but there are no specific restrictions written in that bullet point, so it would seem that it works in dungeons as well.