K-T boundary

Short definition from Wiktionary

Noun

1. 1
   The geological signature of the transition between the Cretaceous period and the Tertiary period 65.5 million years ago.
2. 2
   The geological signature of the transition between the Cretaceous period and the Tertiary period 65.5 million years ago. This transition is marked worldwide by a layer of material that contains a high level of iridium: This transition is marked worldwide by a layer of material that contains a high level of iridium.