Risk mitigation means what you could do in order to minimize the risk. You could minimize the risk either by decreasing the occurence probability or minimizing the impact.
Mitigation of risk is only one choice. Others could be to avoid, to transfer it to somebody else, or simply to accept it.
Just to take as an example the risk of not getting an answer to your question:
-I would have chosen the avoidance strategy, by searching myself (on QAForums or on elsewhere).
- considering the mitigation you could have tried to read the QAForums guidelines about posting, find the right Forum (by the way, this is NOT the right one) and trying to rephrase the question. These could minimize the probability. About the impact...
- The transfer of the risk could be asking somebody else to find out this for you (if you're a team leader or project manager...).
Adding to the above and in short Risk Contigency is where u identify different risks which your project may come across....and Risk Mitigation is one where you identify the possible measures to control all the risks that have been identified in your project.