Definition of a condition: When it is non-existent it necessitates non-existence, but when it is existent, it does not necessitate existence. For example, the non-existence of wudu necessitates the non-exitance of the prayer, but the existence of wudu does not necessitate the existence of prayer (i.e. you don't always need to be praying if you have wudu, but without wudu you cannot pray).