Sanskrit Kosha dictionary gives:-
Śaraṇāgati (शरणागति): surrender
Prapatti (प्रपत्ति): Pious resignation or devotion.
Shri Shailacarya (18th century) uses the words interchangeably in Chapter 18 of Siddhanta Sangraha
- The word ‘śaraṇa’ in the statement ‘śaraṇam prapadye’ implies that ‘śaraṇāgati’ or prapatti is not in need of any other means and can achieve the desired end by itself. This alone should be understood with this expression.
However, sometimes even synonyms have subtle difference between them.
A famous example in Hindi language being - 'dayā (दया) & karuṇā (करुणा)'
dayā (दया) = Sympathy - Mercy
karuṇā (करुणा) = Empathy - Compassion