In sociolinguistics, a language continuum is said to exist when two or more different languages or dialects merge one into the other(s) without a definable boundary.

For example, on both sides of the border between the Netherlands and Germany, the people living in the immediate neighbourhood of the border speak an identical language. They can understand each other without difficulty, and would even have trouble telling just by the language whether a person from the region was from the Netherlands or from Germany. However, the Germans here call their language German, and the Dutch call their language Dutch, so in terms of sociolinguistics they are speaking different languages.

There are many other examples throughout the world.