Why Do They Call It 'Root Beer?'

Root beer is a dark brown sweet beverage traditionally made using the root or bark of the tree Sassafras albidum (sassafras) or the vine Smilax ornata (sarsaparilla) as the primary flavor.
Root beer may be alcoholic or non-alcoholic, and may be carbonated or non-carbonated. Most root beer has a thick foamy head when poured. Modern, commercially produced root beer is generally sweet, foamy, carbonated, and non-alcoholic, and is flavored using artificial sassafras.