Apple cider, pear cider, etc
Cider is an alcoholic drink made by naturally fermenting apple or pear juice. Traditional cider made with pears is commonly called perry.
When a recipe on this site calls for cider, use some good-quality French, British or Irish-type hard cider, with the alcoholic content ranging between about 3 and 6,5 % by volume.
Use either sweet or dry cider, according to your taste.
One good brand, especially suitable to be used in beverage and drink mix recipes of this site, is the Irish Bulmers/Magners, with not too musty or overpowering, yet fruity and refreshing apple taste.
Do not use non-alcoholic cider ("sweet cider") as a replacement, unless it is carbonated. Also cider-type carbonated beverages like the Finnish Golden Cap and Upcider or the Swedish Kopparberg should never be used, as they all have an artificial candy-like and/or cloyingly sweet flavour, not reminiscent of true ciders in any way.
Pictures on left and right: French apple cider bottles.