<p><strong>Noun</strong></p><ul><li>The melting of a significant portion of a nuclear-reactor core due to inadequate cooling of the fuel elements, a condition that could lead to the escape of radiation.</li><li>A quickly developing breakdown or collapse: a bond-market meltdown; the meltdown of a marriage.</li><li>(Informal) A sudden loss of control over one’s feelings or behavior: My toddler had a meltdown when I tried to leave the house.</li></ul>