Omega-9 fatty acids are a family of unsaturated fatty acids.

Two of the most well-known omega-9 fatty acids are oleic acid and erucic acid. Oleic acid is a main component of olive oil and other monounsaturated fats. 
 
Erucic acid is found in rapeseed (canola oil), wallflower seed, and mustard seed.

Unlike omega-3 and omega-6 fatty acids, omega-9 fatty acids are not classed as essential fatty acids (EFA). This is both because they can be created by the human body from unsaturated fat, and are therefore not essential in the diet, and also because the lack of a double bond keeps them from participating in the reactions that form the eicosanoids.

References

Lipomics. Mead acid. Retrieved on February 14, 2006.