You see them at all the grocery stores this time of year: bottles of apple cider. So imagine my surprise when I read in Garner's Modern American Usage that "apple cider" is technically redundant. Cider is defined as "juice pressed from apples." It comes from the Late Latin word for "strong drink."
"Apple cider" is an example of marketing influencing the language. Some drink makers call juice from other fruits cider (e.g., blueberry cider, cherry cider), so some manufacturers who make traditional cider feel the need to point out that their juice is from apples.
"Apple cider" now joins "chai tea" (which technically means "tea tea," since "chai" is the Hindi word for "tea") as a phrase that is technically redundant, but often needed so consumers understand what they're buying.
UPDATE: Ngram graph added 10/14/2011:
Mignon Fogarty is the author of Grammar Girl's 101 Misused Words You'll Never Confuse Again. Follow her on Twitter or Facebook.