A group of languages or
language families that have features in common, arising by shared influence rather than descent. The term is German for "language
league".
The common features are called areal or geographic features: they arise by convergence. Languages that are not intrinsically very similar influence each other into adopting similar tricks of phonetics, syntax, or whatever.
One classic Sprachbund is in the Balkans, where Romanian, Bulgarian, and Albanian have the unusual feature of a suffixed definite article. Romanian is Romance but no other Romance language does this; Bulgarian is Slavonic but no other Slavonic language does this: so it looks as if the peculiarity arose in Albanian and spread to the other two, which are only very distantly related.
Another Sprachbund is in South-East Asia. Superficially, Thai and Vietnamese look like Chinese, especially in their syllabic structure and tonality; for a long time it was believed there was a genetic relationship between them. Now they are regarded as very, very different genetically, and have acquired their common features by areal convergence.
In Indo-European studies, features once thought to indicate the history of divergence of different branches, such as the split between centum and satem languages, are now understood differently. By being in the same area, different groups acquired similar structures. The Germanic and Slavonic languages were in contact, and they were also in contact with the Finno-Ugrian family for a long time. This occasioned much borrowing.
Another possible Sprachbund is the Altaic and Ural-Altaic "families". The Turkic, Mongolian, and Manchu-Tungus families share a great deal of similarity in many ways; and so also do the Uralic languages such as Finnish and Hungarian. It is tempting to say they all belong to an Altaic family, and above that a Ural-Altaic family comprising the whole lot. But detailed examination of the evidence of connexion shows that the resemblances might have been caused by widespread borrowing.
All languages borrow, but mostly the borrowed words are obviously distinct from the native stock. It is when the scale of assimilation is so large that it starts to look (falsely) like a deeper relationship that the concept of Sprachbund is invoked.