One in a series of rows or levels (4)
I believe the answer is:
tier
(Other definitions for tier that I've seen before include "Level within hierarchy" , "Wedding cake feature" , "Row in theatre" , "Row, layer" , "Plague" .)
I believe the answer is:
tier
(Other definitions for tier that I've seen before include "Level within hierarchy" , "Wedding cake feature" , "Row in theatre" , "Row, layer" , "Plague" .)