
Apple cider (unsweetened) is a low-alcohol drink of apple juice fermented close to dry.
Fermentation is what separates it from juice: dryness, a little alcohol and a savory depth that no unfermented apple brings. A non-alcoholic route covers the fruit and the fizz and leaves the depth out.
Unfermented cider (US)
In the US "apple cider" often means cloudy unfermented juice with no alcohol; this card is about dry fermented cider with an ABV. Build a highball on the juice and the drink loses its strength and comes out sweet.
Unsweetened cider is fermented apple juice and nothing else, which puts it nearer a dry white wine than anything in the soft-drink aisle. The sugar the apples came with has gone into alcohol, so what is left is acidity, tannin from the skins and a fruit note that is drier than the smell suggests. That dryness is what makes it useful. It lengthens a drink and brings apple without bringing sugar, and any recipe that assumed a sweet cider will come out sharp on it.
Once opened it loses carbonation and freshness; keep refrigerated and use within a few days.