Vibration Theory & Terminology: Order
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Order refers to how many times an event occurs during one revolution of a rotating component. For example, a tire with one high spot would create a disturbance once for every revolution of the tire. This is called first-order vibration.
An oval-shaped tire with 2 high spots would create a disturbance twice for every revolution. This is called second-order vibration. Three high spots would be third-order, and so forth. Two first-order vibrations may add or subtract from the overall amplitude of the disturbance, but that is all. Two first-order vibrations DO NOT equal a second-order. Due to centrifugal force, an unbalanced component will always create at least a first-order vibration.