What is the difference between IMUs and other devices that have accelerometers and gyroscopes (smartphones, tablets, sensewear etc.)?
The general definition of IMU says that any device that has accelerometer, gyroscope and magnetometer/GPS packed together is an IMU. 


Where did you find that definition?

It's true that most systems classified as IMUs have most of those sensors, but there are variations.

In general, an IMU is a device designed to track the position and orientation of an object &mdash; along with their derivatives such as velocity and angular rates.

The data is provided relative to some reference. If the reference is an absolute one, such as the orientation of the Earth, it is referred to as an absolute IMU. Otherwise, if the the reference is arbitrary (such as the position/orientation at power-up), then it is a relative IMU.

Accelerometers and rate gyros give relative information only, and are subject to drift in various forms. An IMU that includes only these sensors can only be a relative IMU. Magnetometers provide zero-drift readings of orientation, while GPS can give a zero-drift measurement of position. IMUs that include these sensors can provide absolute data.

It takes a significant amount of software to take the raw data from all of the available sensors and "fuse" it together to come up with the best possible estimate of position and orientation. The Kalman Filter is one popular approach.


  Then why aren't other devices that have the same sensors called IMUs?


Because "being an IMU" is not their primary function. They may not even include the appropriate software to combine the data from the various sensors into a full position+orientation solution.