Wiktionary
tone sandhi
n. (context linguistics English) change of tone in tonal languages when some tones come together
Wikipedia
Tone sandhi
Tone sandhi is a phonological change occurring in tonal languages, in which the tones assigned to individual words or morphemes change based on the pronunciation of adjacent words or morphemes. It usually simplifies a bidirectional tone into a one-direction tone. It is a type of sandhi, or fusional change, from the Sanskrit word for "joining".