 Solution

A mass extinction is a geologically brief, geographically widespread loss of a large fraction of species across multiple major groups, substantially exceeding background extinction rates. Severity, global extent and rapidity distinguish it from the disappearance of a local population or the ordinary loss of individual lineages. A threshold near $75\%$ of species is often used operationally, but the qualitative definition does not depend on one universal numerical cutoff. The duration is short on geological timescales, and need not mean that every species disappeared simultaneously in a single day.