Protective Spirit (an Oath of Redemption feature) says:
you regain hit points ... if you end your turn in combat with fewer than half of your hit points remaining
What qualifies as a combat in order to heal in this way?
According to the Player's Handbook in the section The Order of Combat at the beginning of Chapter 9 (p. 189, emphasis mine):
A typical combat encounter is a clash between two sides, a flurry of weapon swings, feints, parries, footwork, and spellcasting. The game organizes the chaos of combat into a cycle of rounds and turns.... Once everyone has taken a turn, the fight continues to the next round if neither side has defeated the other.
The flavor text that introduces the same chapter also describes combat as:
...a brief skirmish or an extended conflict in a dungeon or on the field of battle.
By definition, combat is when creatures are fighting. It also happens to be when the game is generally broken up into rounds and turns. So features that mention turns in combat, as the feature you quoted, necessarily work when you are fighting with opponents.
Basically, it's just the common English reading of the word. In this case, there is a passage that ensures the common English reading is indeed the correct interpretation.
Note that a DM might rule that you're still "in combat" when all enemies are defeated and you or your allies are bleeding out or otherwise still dealing with the repercussions of battle. For example, if the DM continues to track initiative after all enemies are defeated in order to play out death saving throws on the party side, it would be reasonable for them to count that as a continuation of combat and allow the feature you quoted to be used.