The OED gives a definition for “Nabi” but not for “Rasul”.
Messenger; a prophet sent by God to deliver a book of scripture.
Term Variants: Rasool, Rasul, Rusul
According to Islamic doctrine, the Prophet Muhammad (PBUH) is the last Rasul of Allah, and the Quran is the culmination of God’s message. This is a central tenet of Islamic belief.
Rasul (messenger) is one of two terms used in the Quran to refer to the Prophets of Islam; the other term is “nabi”. There is some debate over the distinction between the two terms. The Oxford English Dictionary, for instance, maintains that ‘rasul’ is a major prophet who brings a new revelation, whereas ‘nabi’ refers to minor prophet who does not deliver a scriptural revelation. The Oxford Dictionary of Islam states that the Quran uses the two terms interchangeably, but it is nevertheless important to be mindful of the distinction and to parse the context to see if a distinction is in fact being intended.