This article states that
If an object is being forced to vibrate at its natural frequency, resonance will occur
I can understand how this definition applies to a playground swing: if person A pushes person B, who is on the swing, in a certain way, the swing will swing higher.
If I'm not mistaken, resonance can also occur when an object is caused to vibrate at a multiple of its natural frequency.
If I were to flick the tallest mass on the left side, the corresponding mass on the right side will vibrate with the same frequency. This is an example of forced vibration. However, no matter how much "frequency" I give the mass on the left side, the mass on the right side will still resonate. How does this make sense if resonance can only occur with multiples of an object's natural frequency?