Terms & Definitions

BYA → Billion Years Ago

MYA → Million Years Ago

KYA → Thousand Years Ago

YA → Years Ago

The fundamental planetary state of Earth changes.

Largest official division of geologic time, spanning hundreds of millions to billions of years.

Defined by fundamental changes in Earth’s atmosphere, biosphere, or planetary conditions.

Examples: Formation of Earth, Appearance of life

Planetary state persists, but dominant global life–environment regime shifts.

Subdivision of an eon, marking major transitions in Earth’s geology, climate, or life.

Examples: Rise or fall of major life groups, Long-term climate regimes

Planetary state persists, global life–environment remains, but impactful reorganizations; often bounded by global-scale events.

Subdivision of an era; significant shifts in life forms, climate, or geology.

Examples: Mass extinctions, Major climate events

Planetary state persists, global life–environment remains, not global-scale transformative, but some noteworthy finer-scale changes.

Subdivision of a period, marking refined or smaller-scale changes in Earth’s history.

Scope: Significant, often regional, though globally co-relatable.

Examples: Glacial–interglacial cycles, Faunal turnovers within established groups

Planetary state and global life–environment remain unchanged; records short-duration, finer-resolution changes for precision.

Smallest subdivision of an epoch, defined by subtle but identifiable shifts in climate, sea level, sedimentation, or biostratigraphy. 

These changes are often regional.