0 plus (-50) plus 500? On reflection, that's the product of a higher power (4,3)
I believe the answer is:
opus dei
'product of a higher power' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'0 plus 50 plus 500? on reflection that's' is the wordplay.
'0 plus 50 plus' becomes 'opus' (I can't explain this - if you can you should give a lot more credence to this answer).
'500?' becomes 'D' (Roman numeral for 500).
'on reflection' is a reversal indicator.
'that's' becomes 'ie'.
'ie' in reverse letter order is 'ei'.
'opus'+'d'+'ei'='OPUS DEI'
'the' is the link.
(Other definitions for opus dei that I've seen before include "Religious group" , "ritual worship" , "RC organisation which maintains various universities" , "prayers" , "Group of believers" .)