The `flutter.ndkVersion` property is defined in the `build.gradle` file of a Flutter project, specifically in the `android` block.

Here's an example of how the `flutter.ndkVersion` property is defined in a typical `build.gradle` file:

android {
  ...
  flutter {
    ...
    // Define the NDK version
    flutter.ndkVersion = '21.0.5518470'
    ...
  }
  ...
}

In this example, the `flutter.ndkVersion` property is set to `21.0.5518470`, which is the version of the Android NDK that Flutter uses by default.

You can modify this value to use a different version of the Android NDK, if necessary. For example, if you want to use version 22.0.5521102 of the NDK, you can update the `flutter.ndkVersion` property accordingly:

android {
  ...
  flutter {
    ...
    flutter.ndkVersion = '22.0.5521102'
    ...
  }
  ...
}

Note that changing the `flutter.ndkVersion` property can affect the behavior of your Flutter app, and may require you to make additional changes to your project's build configuration.