# is

### Description

Only used in [declarations](https://docs.atfinity.io/rule-language/declarations), `is` assigns a name or identifier to the specified [ontology](https://docs.atfinity.io/guides/glossary/ontology). This name or identifier can then be used in the same rule to refer to that ontology, when you want to access [information](https://docs.atfinity.io/guides/glossary/information) on it to use in the condition, or to add a [field](https://docs.atfinity.io/rule-language/operators/special-operators/broken-reference) on a [document](https://docs.atfinity.io/guides/glossary/document).

### Example

```
p is Person
```

A declaration with `is` is needed before you can have any expressions using `p`, like: `p.domicile != ch`.