Who has given root canal isthmus classification?
Kim et al Classification
Root apex cross-section
Narrow ribbon-shaped connection that contains pulp or pulpally derived tissue - ISTHMUS
According to Kim et al classification; there are 5 types:

Type I : Incomplete isthmus between 2 canals.
Type II : Well defined, complete isthmus between 2 canals.
Type III : Very short, but complete isthmus between 2 canals.
Type IV : 3 or more canals with the complete or incomplete isthmus.
Type V : 3 or more canals without any visible connection.
Kim
Schielder
Krasner and Rankow
Vertucci