| A | B |
|---|
| Fossil | The imprint or hardened remains of a plant or animal |
| Original remains | A fossil that is the actual body or body parts of an organism. |
| Ice core | A tubular sample that shows the layers of snow and ice that have built up over the years. |
| Relative age | The age of an event or object in relation to other events |
| Index fossil | A fossil of an organism that was common, lived in many areas, and existed only during a certain span of time. |
| Absolute age | The actual age in years of an event or object. |
| Half-life | The length of time it takes for half of the atoms in a sample of a radioactive element to change from an unstable form into another form. |
| Uniformitarianism | A theory stating that processes shaping Earth today, such as erosion and deposition, also shaped Earth in the past, and that these processes cause large changes over geologic time. |
| Geologic time scale | The summary of Earth’s history, divided into intervals of time defined by major events or changes on Earth. |
| Eon | The largest amount of time in divisions of geologic time |
| Era | Division of time within an eon |
| Period | Division of time within an era |
| Epoch | Division of time within a period of time |
| Unicellular organism | An organism that is made up of a single cell. |
| Multicellular organism | An organism that is made up of many cells. |
| Mass extinction | One of several periods in Earth’s history when large numbers of species became extinct at nearly the same time. |
