The Urim is one of two enigmatic items that was placed in the breastplate of the high priest, the other one being the Thummim. We don't know what they were or what they were made of but the Urim and the Thummim were somehow used to consult God (Num 27:21, 1 Sam 28:6).
The name/word Urim comes from ('or 52) meaning to be or to become light, shine, give light. Derivation ('or 52a) means light, and ('ur 52d) means flame. ('ora 52b) is a feminine form of the word light (as such also used to mean joy) and also the name of a certain herb ('ora as used in 2 Ki 4:39); possibly a very spicy or otherwise inflaming herb, or simply endowed with bright blossoms, etc. TWOTOT lists the Urim as 52e. The regular word for star is (ma'or 52f) and literally means luminary.
Urim is a regular plural and means Lights or Flames.
Related names are Ur and perhaps Aaron and the Hebrew word for Nile.