A keyboard with keys that are scaled to about 75% of the dimensions of a standard keyboard. Mini keys are used on some synths to save space and weight, and because they are somewhat less expensive to manufacture. Performers with large hands generally dislike mini keys, but some other performers prefer them (or at least don't mind them) because they can reach across a larger span, and see more of the keyboard without moving their eyes. Velocity and aftertouch can be more difficult on a mini keyboard because of the short throws. (Most mini keyboards don't have these features anyway.)
The photo below of a Wurlitzer organ illustrates the difference between standard size keys (the lower two manuals) and mini keys (the top manual).