Style of painting round region (2,3)
I believe the answer is:
op art
'style of painting' is the definition.
(I've seen this before)
'round region' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'region' becomes 'part' ('part' can be a synonym of 'region'**).
'o'+'part'='OP ART'
(Other definitions for op art that I've seen before include "Illusory works" , "Illusory graphic work" , "confusing images" , "Style of optical illusionism popular in the 1960s" , "modern paintings" .)