Regular Expression (RegEx)

Regular Expression, often abbreviated as RegEx, is not an exclusive term to React JS but a powerful tool used primarily in JavaScript and various other programming languages for matching and manipulating strings.

A regular expression is a sequence of characters that forms a search pattern. This pattern can be used to search, edit, or manipulate text or data. This search pattern can be used for text search and text replace operations.

In the context of React or JavaScript, we could use regex for form validations, splitting strings, replacing substrings, etc.

For instance, we can check if an input string follows the standard email format or not by using a regular expression as shown below:

```javascript
const email = "test@example.com";
// Regular expression for validating an email
const regex = /^[\w-]+(\.[\w-]+)*@([\w-]+\.)+[a-zA-Z]{2,7}$/;

// Use the JavaScript .test() method to test the regular expression against the email string
if(regex.test(email)) {
console.log("The email is valid");
} else {
console.log("The email is not valid");
}
```
In this example, if the `email` string adheres to the email standard format, "The email is valid" will be logged to the console, else "The email is not valid" will be logged.