Walkstation

For example, the MU15 has a keyboard with keys that are approximately 6 mm apart (from one white key to the next adjacent white key). That is approximately four times more closely packed than the keys on a standard piano (standard piano key spacing is approximately 24mm from one white key to the next).
Although not identified as a walkstation, the Yamaha HandySound HS-200 is a similarly sized portable unit that also has a built in speaker.
Etymology
The term was coined by Yamaha to build upon the Walkman phenomenon started by Sony."Walkstation" is a portmanteau of workstation and walk.
Examples of walkstations
Yamaha MU5
Yamaha MU10 (early walkstation, no display)
Yamaha MU15 (early walkstation, with display)
Yamaha MU20, half-rack unit, 1995
Yamaha QY8 (early walkstation, only has 8 MIDI channels, 1994); Owner's manual

Only responds to MIDI channels 1-7 and channel 10.