heather
From The Oxford Companion to Beer
(Calluna vulgaris) is a widespread, small bush-like plant that grows on dry, acidic soils. In the northern hemisphere it usually flowers in August and has a strong and pleasant aroma during flowering. Both young shoots of heather and the very aromatic flowering shoots may be used for bittering and flavoring beer. In many areas of Europe, heather was a common component of gruit, a mixture of herbs that was widely used in beer in the centuries before hops became predominant.
Bibliography
This definition is from The Oxford Companion to Beer, edited by Garrett Oliver. © Oxford University Press 2012.