It somewhat depends on the usage in your particular field. In most cases I have seen, the order refers to a multiple of a fundamental frequency. In your case that would probably be the once per rev engine speed. So a second order vibration would be a vibration with a frequency of 2X of engine speed. In the vibration analysis world, the order of the vibration is a first indicator of a source for the vibration. For example, a first order (or as I am used to saying, a 1E) is a very good indication of a shaft imbalance.
I found this page related to automotive engines that probably will give you what you need.
http://www.vibratesoftware.com/html_help/html/Diagnosis/Engine_Speed_Related.htm