Greetings christerra99 - as oginme alluded to, your question has a number of different meanings. So, I will assume your question is in regard to a recipe where the exact ingredient is not that important. Instead, the recipe author offers a "general" description for one of the ingredients.
So, I visited The Center of the Universe Brewing Co. and found this description:
http://www.cotubrewing.com/homebrewing/malt-profiles/
"Crystal/Caramel Malts:. These malts add the a beers color, mouth feel, and imparts a nutty or caramel flavor. The intensity depends on the SRM ratings from 10-120L."
So with that in mind, Oginme's response is on target as 55 SRM would be exactly in the middle between 10 and 120, or "medium crystal". 120 - 10 = 110 / 2 = 55