And just to clarify, it is not a ranking factor nor mandatory you put the keyword in the meta description. I personally do because it is statistically proven that websites that do, typically rank higher in the search results. In the matter of Keyword Density, I have seen 0.5% rank #1 as well as over 2.5%. However once you start taking it higher, at about 5% it can negatively impact you for keyword stuffing.