Pedometer

[pee-do-mee-ter] –noun

  1. The distance at which parents consider their young offspring safe from the nearest pedophile. Unusual among Système International units of measure in that the pedometer varies depending on the material through which you measure, a pedometer is equal to approximately 1.828 meters (or 6 feet) when measured through soil, or 55,758,006,000 meters when measured through the vacuum of space. Abbreviation: pedo
  2. An instrument that gauges the distance to the nearest pedophile.

Also, British, paedometre.