the long thin part of a plant, from which leaves, flowers, or fruit grow [= stalk]
the long thin part of a wine glass, vase etc, between the base and the wide top
the narrow tube of a pipe used to smoke tobacco
having a long stem, a short stem etc:
long-stemmed wine glasses
the part of a word that stays the same when different endings are added to it, for example 'driv-' in 'driving'