Central points (4)
'central points' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for foci that I've seen before include "fixed points" , "targets" , "Points for attention" , "Central points (of the greatest activity)" , "Centres" .)
(Other definitions for loci that I've seen before include "Particular positions" , "Particular places" , "locations" , "sites" , "Points where something occurs; coil (anag.)" .)