A walkstation is a portable music synthesizer that can often be programmed with various musical sequences.
Typically walkstations are small battery powered units with small size keys that are ideal for the smaller hands of young children. 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.
The term was coined by Yamaha to build upon the Walkman phenomenon started by Sony. "Walkstation" is a portmanteau of workstation and walk.