Actually, the word beer has been historically used for all fermented beverages. When referring to medieval texts for brewing and other recipes it can take a while to sort out what drink they're referring to.

Beer was only legally referred to as a fermented beverage flavoured with hops as recently as the 18 century, when in Britain it became illegal to make it from anything else.

http://www.merriam-webster.com/dictionary/beer

Main Entry: beer
Pronunciation: \ˈbir\
Function: noun
Etymology: Middle English ber, from Old English bēor; akin to Old High German bior beer
Date: before 12th century
1 : an alcoholic beverage usually made from malted cereal grain (as barley), flavored with hops, and brewed by slow fermentation
2 : a carbonated nonalcoholic or a fermented slightly alcoholic beverage with flavoring from roots or other plant parts
3 : fermented mash
4 : a drink of beer


http://www.answers.com/topic/beer
        
a.A fermented alcoholic beverage brewed from malt and flavored with hops.
b.A fermented beverage brewed by traditional methods that is then dealcoholized so that the finished product contains no more than 0.5 percent alcohol.
c.A carbonated beverage produced by a method in which the fermentation process is either circumvented or altered, resulting in a finished product having an alcohol content of no more than 0.01 percent.
2.A beverage made from extracts of roots and plants: birch beer.
3.A serving of one of these beverages.
[Middle English ber, from Old English bēor, from West Germanic, probably from Latin bibere, to drink.]

http://www.thefreedictionary.com/beer

beer   (bîr)
n.
1.
a. A fermented alcoholic beverage brewed from malt and flavored with hops.
b. A fermented beverage brewed by traditional methods that is then dealcoholized so that the finished product contains no more than 0.5 percent alcohol.
c. A carbonated beverage produced by a method in which the fermentation process is either circumvented or altered, resulting in a finished product having an alcohol content of no more than 0.01 percent.
2. A beverage made from extracts of roots and plants: birch beer.
3. A serving of one of these beverages.

[Middle English ber, from Old English b or, from West Germanic, probably from Latin bibere, to drink; see p (i)- in Indo-European roots.]
The American Heritage® Dictionary of the English Language, Fourth Edition copyright ©2000 by Houghton Mifflin Company. Updated in 2003. Published by Houghton Mifflin Company. All rights reserved.