To unbalance is to make unsteady or uneven. A sudden gust of wind might unbalance you when you're on a sailboat — don't forget to wear a life jacket!
Throw something off balance, and you unbalance it. Tickling someone who's in a one-legged yoga stance will unbalance them, and leaning too far to one side will unbalance a new bike rider. You can also call this state of being out of kilter or wobbly unbalance: "The unbalance of the tightrope walker made me glad there was a net underneath him!" And figuratively, unbalance can also mean "disturb or upset."