Microcontrollers include peripheral devices like serial ports, I2C, SPI and so on that may not be included in a more general purpose microprocessor. In fact, microprocessors may not include any peripheral devices at all. Those devices are in separate chips hung on a bus.
The distinction gets a bit blurry when a conventional microprocessor, like the Z80, is upgraded with peripherals and moves into the microcontroller category.
Considering that our microprocessors have more capability than mainframes of the '80s, when do we drop the 'micro' part?