An encounter is any situation where choices matter. Combat encounters involve initiative and attacks. Non-combat encounters involve social interaction, exploration, or problem-solving.
Examples include fighting monsters, persuading a guard, sneaking through a fortress, or navigating a collapsing bridge.
Almost anything. D&D isn’t limited by predefined options. If you can imagine it, you can attempt it.
Rules exist to support creativity, not restrict it. When no rule exists, the DM improvises — that’s part of the magic of the game.