Answer:
The term used to describe how dialects tend to merge over dialect boundaries is "dialect continuum". This refers to a series of language varieties spoken across a geographical area such that neighboring varieties are mutually intelligible, but the differences accumulate over distance so that widely separated varieties may not be. As neighboring dialects come into contact with one another, they tend to merge and align their dialects with one another. This phenomenon typically occurs in long-settled agrarian populations, and dialectologists map the variation of various language features across a dialect continuum, drawing lines called isoglosses between areas that differ with respect to some feature.
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