1. An alcoholic beverage made by brewing and fermenting cereals, usually malted barley, and flavored with hops and the like for a slightly bitter taste
2. Any of various beverages, alcoholic or not, made from roots, molasses or sugar, yeast, etc. (e.g. root beer)
3. An individual serving of beer; a glass, can or bottle of beer
The origin of beer is Old English beor ("strong drink, beer, mead"), which is of uncertain and highly-disputed origin. It may have been borrowed into the Germanic branch of Proto-Indo-European at some point because there is a native Germanic word for the drink that evolved into ale. If so, it was probably taken from Vulgar Latin biber into West Germanic in the 6th century. Another theory is that it evolved organically from Proto-Germanic *beuwoz-, from *beuwo- ("barley").
One thing that is certain is that beer has been around for a long, long, long time. It has even been suggested that it was the reason for the advent of agriculture.*
*A few articles on the subject: The Independent, Miller-McCune, LiveScience