Anonymous Function

In JavaScript and thus in React JS, an anonymous function is a function that is defined without a name. Unlike named functions, anonymous functions are not accessible after their initial creation. These functions are often used for their practical use-cases in callback functions and functional programming methods.

Due to their lack of a name, anonymous functions provide no information to the user about their functionality and intention, which is why they are primarily used for on-the-spot functionality where a function is required.

Here is an example of an anonymous function in React (often used in event handlers):

```
<button onClick={() => { console.log('Button clicked!') }}>Click me</button>
```

In this code, the `onClick` event handler is using an anonymous function to log 'Button clicked!' in the console when the button is clicked. The anonymous function is `() => { console.log('Button clicked!') }`. It doesn't have a name, but it does have a function: to print a message to the console when activated. This is a very common usage of anonymous functions in React.