The Collaborative International Dictionary of English v.0.48:

blip \blip\ n.
   1. (Radar) a spot of light on a radar screen, showing the
      position of a reflecting surface, such as an airplane or
      ship.

   Note: this sense is also used metaphorically to mean a barely
         perceptible object;, Kennedy's candidacy was a mere
         blip on Humphrey's radar screen until he won the West
         Virginia primary. This is the probably origin of sense
         3.

   Syn: radar echo, radar target.
        [WordNet 1.5]

   2. a short upward or downward deviation from a trend line on
      a graph, especially in a plot of some variable, such as an
      economic variable, against time; as, a brief blip upward
      in the unemployment rate.
      [PJC]

   3. something small or insignificant.
      [PJC]

   4. a brief interruption in the continuity of a recorded or
      transmitted signal; as, there is a blip on my CD of
      Beethoven's ninth.
      [PJC]

WordNet (r) 3.0 (2006):

blip
    n 1: a sudden minor shock or meaningless interruption; "the
         market had one bad blip today"; "you can't react to the
         day-to-day blips"; "renewed jitters in the wake of a blip
         in retail sales"
    2: a radar echo displayed so as to show the position of a
       reflecting surface [syn: blip, pip, radar target]