Two different contexts here, one an international measuring system, and the other describing an ignition system:

  1. Literally "Systeme Internationale", the international system of measurement. It uses units of measurement such as the second, the metre, the kilogramme etc. Commonly referred to as the "metric system". Visit a related website
  2. Spark Ignition, describing how the air/fuel mixture is fired in a petrol engine - it uses a spark to ignite the mix.

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