"one who is sent", an apostle, a messenger. Muslims distinguish between rasul and nabi. The former is supposed to be one who brought a scripture, thus a messenger, while the latter is simply a prophet. Interestingly, however, in Maryam 19:51, Moses is called "a rasul, a nabi", while two verses later Aaron is called just "nabi". But in Ta Ha 20:47, Allah addresses both of them in the dual number and tells them to go to Pharaoh and say: "We are two rasuls of thy Lord". Thus, Aaron is here called a rasul along with Moses. Further, in al-Baqarah 2:285, the Qur'an told the Muslims to believe in "His books and His messengers", while in al-Baqarah 2:177 they are told to believe in "the Scripture and the prophets". Some Muslims believe that these two terms are used interchangeably.
However, the imams and mufassirun (interpreters) infered that specifically speaking there is a difference whereas generally speaking, the attribute is interchangeable between nabi and rasul, the rasul being higher than the nabi, because the rasul brings a divine legislation with him, whereas the nabi follows the legislation of the previous rasul. Thus, these doctors believed that the rasul is also a nabi, but a nabi may not be a rasul.
Interestingly, it is said that Imam Suyuti said in his collection of hadith exclusively devoted to the subject of angels, al-Haba'ik fi akhbar al-mala'ik that the rasul receives divine communication from Gabriel, whereas the nabi receives communication from a lesser angel.
Further reading:
A very detailed discussion of the terms rasul and nabi as found in the Qur'an is given in Arthur Jeffery's The Qur'an as Scripture, Part II, particularly this section.