What exactly is a "Sensor"
Tricky question... But the answer is quite simple. It's the purpose of the sensor. If it's built to detect (or sense) changes of things (like gravity or light) and give an output at the moment it detects a change it's a sensor.

If it's built to connect to other devices or to take pictures when you click a button it's a sensor but it's not considered to be a sensor by most people and manufacturers.

Let's take a camera as example. In your smartphone the camera is only running when you start it that's why it's not considered to be a sensor even though a camera uses a CMOS sensor. Simply because its normal operating state is turned off and not to detect changes.