Resonance
Definition
When you push something at exactly its natural rhythm, small pushes add up into huge oscillations.
Technical definition
Large-amplitude oscillation driven at or near a system's natural frequency, where each driving cycle adds energy in phase with the existing motion. Amplitude is limited only by damping.
Example
Push a child on a swing in rhythm and tiny pushes build a huge arc; push at the wrong rhythm and you kill the motion. An opera singer shattering a wine glass works the same way — the note matches the glass's natural frequency.
Resonance is not the same as simple vibration. Everything vibrates when hit, but resonance is the special match between driving frequency and natural frequency that amplifies the response. A bridge does not need hurricane winds to fail — it needs gusts timed to its own natural sway, which is why soldiers break step when marching across bridges.