The Collaborative International Dictionary of English v.0.48:

Obelus \Ob"e*lus\ ([o^]b"[e^]*l[u^]s), n.; pl. Obeli
   ([o^]b"[e^]*l[imac]). [L., fr. Gr. 'obelo`s, prop., a spit.]
   (Print.)
   A mark [thus ---, or /]; -- so called as resembling a needle.
   In old MSS. or editions of the classics, it marks suspected
   passages or readings.
   [1913 Webster]