i have seen it mentioned here a couple of times. google doesn't say what it is. can someone tell me it does and the purpose of it? thanks.

It probably means you can allocate different sounds to four different zones of the keyboard - the bottom 2 octaves to a bass or synth/bass mix, the middle 3 octaves to a piano and the top two octaves to a sound pad and some voices or horns, or percussion. Each zone can be programmed to respond to sustain pedal or not. Each zone can have a different volume level and effects mix. Each zone may use internal sounds from the keyboard you're playing or - using MIDI - trigger sounds from another keyboard, module or computer....or any combination of the above.

At least I think that's what a zoned keyboard is. I haven't actually got one. But my DP does have a split facility which gives it 2 zones, I suppose.

... also, zones can be overlapped.

You could layer, for example, four sounds to cover the entire keyboard, and control the volume setting (and many other parameters) for each zone.

Two different concepts:

MIDI Controller = ability to trigger and control external sounds (sounds that are not built into the keyboard itself; i.e. sounds in a computer, iPad, sound module, or some other keyboard)

Four zone = ability to split the keyboard into four areas (which may or may not overlap) for the purpose of triggering different sounds from different areas of the keyboard (or technically, in some cases, you may create a zone that is not assigned to ANY keys of the keyboard, you may want to send it MIDI data for some other purpose)

A keyboard that can be split once would have two zones, i.e. above and below the split point. A keyboard that can be split twice has three zones, like the Nord Stage 2 or Hammond SK1. And so on. Kawai MP6, Roland RD-700, Yamaha MOX are some examples that have four zones. Korg Kronos and Kurzweil PC3 series have 16 zones.

edit: P.S. -- that's a general conceptual overview, but specific implementations or how these things are defined by manufacturers can vary a bit. For example, Casio says the PX3 is 4-zone, and in a sense it is, but it only supports one split point, so zones must overlap, as there are only two definable key regions.

.... That's a fail IMHO

The PX-3 does in fact only have one split point. It is still considered 4 zones as each of the zones can independently control internal or external MIDI devices. There are many Roland products that share this same terminology having Upper and Lower zones.

The new PX-5S has full independence of each of the zones. They can overlap, split, velocity switch, layer...there aren't any rules.