| A | B |
|---|
| Relative age | The age of a rock compared to the ages of other rocks. |
| Absolute age | The age of a rock given as the number of years since the rock formed |
| radioactive decay | The process in which the nuclei of radioactive elements break down, releasing fast-moving particles and energy |
| radioactive dating | The process of determining the age of an object using the half-life of one or more radioactive isotopes |
| era | One of 3 long units of geologic time between the Precambrian and the present |
| period | One of the units of geologic time into which geologists divide eras |
| invertebrate | An animal without a backbone. |
| vertebrate | An animal with a backbone |
| amphibian | A vertebrate whose body temperature is determined by the temperature of its environment, and that lives its early life in water and its adult life on land |
| reptile | A vertebrate whose temperature is determined by the temperature of its environment, that has lungs and scaly skin, and that lays eggs on land |
