Definition from Wiktionary, the free dictionary
RIP or R.I.P.
- Latin: requiescat in pace, 'may he rest in peace'
- Rest in peace, the identically abbreviated English translation
- When he died he received hundreds of letters signed with RIP at the end.
- Routing Information Protocol (RIP), a dynamic routing protocol used in local and wide area networks
Usage notes 
- Pronounced letter by letter, not as rip.
- Typically found as epitaph on tombstones.
- Also used as an epithet, when referring to a deceased person, as in “This university was founded by Thomas Jefferson, RIP.”
- Can be used as an imperative verb: "She died in a car accident last week. RIP, Christy M."
- The phrase is never used in reference to sleep / a night’s rest; it refers the speaker’s wish that a deceased person is at peace. It is often used in memorials to deceased persons and found on tombstones.
- The stereotypical representation of a grave (e.g. in Halloween decorations, cartoons, etc) is a tuft of land with an upright tombstone with RIP engraved on it.
Related terms 
rest in peace
RIP (also: R.I.P.)