1. of, relating to, or denoting the earliest eon of Earth's history, from about 4.6 billion to 2.5 billion years ago
Usage: geology; often capitalized as Archean

1. the Archean eon; the earliest division of geological time
Usage: geology; often capitalized as the Archean

Examples

• Archean rocks are among the oldest on Earth.
• The Archean eon saw the formation of the earliest continents.
• Geologists study Archean fossils to understand early life.
• During the Archean, Earth's atmosphere lacked free oxygen.
• Archean greenstone belts contain valuable mineral deposits.