distinguished professor
An unofficial honorary title granted by a university or institution to recognize exceptional contributions, not a standard academic rank.
In recognition of his many years of research, he was awarded the title of Distinguished Professor.
名誉教授 (professor emeritus) is a common official title for retired professors, while 特別栄誉教授 is an unofficial, institution-specific honor.
Compound of 特別 (special), 栄誉 (honor), and 教授 (professor). The term is a modern institutional coinage with no classical origin.