Vermouth, from the German, 'vermut' (wormwood).
Vermouth is an aromatised wine, which means, that unlike a fortified wine in which a spirit is added to significantly boost the volume of alcohol, additives are done purely to modify the flavour. It is heavily flavoured with sugar, herbs, roots, flowers, and spices and contains up to 19% alcohol.
Pronounced 'ver-MOOTH', the name comes from the German wermut ("wormwood"), which, before it was declared poisonous, was once the principal flavouring ingredient.
There are several types of this wine, the most popular being dry white vermouth, commonly thought of as French and developed in the early 19th century by Joseph Noilly. The sweeter, darker type of vermouth is sometimes called Italian vermouth and is excellent on the rocks with a twist and made in other countries including the United States.
It's served as an aperitif and used in non-sweet cocktails like martinis. The reddish brown sweet vermouth (which is coloured with caramel) is also served as an apéritif as well as used in slightly sweet cocktails such as the Manhattan. A third style called Bianco is white and slightly sweet, and not as popular as the other two.
Noilly Prat and Martini & Rossi are well-respected brands. Lillet an aperitif from Bordeaux is authentic and highly recommended. Other top brands include, Cinzano & Dubonnet.
Many people make martinis with just gin and olives or Punt e Mes, where this bitter vermouths cousin turns a martini into a red martini.