Ductless ac mini split systems - what is being split?

Split system indoor (Evaporator) and outdoor (Condenser) is why the call it a split system, package units like rooftops have everything in a central location. Mini splits have no ductwork to run etc

I am mostly reiterating what has been said but, just in case you needed a little more info on the mini-split air unit. A conventional A/C unit has all of the cooling and evaporative parts in one unit that sits outside of your house and is connected to the blower in your furnace to deliver the cooled air throughout your home via duct work and is controlled by your home thermostat. A mini-split unit is split into 2 parts. It has a small condenser unit that sits outside of your house. A hole is drilled through the wall. A thin tube is connected to the unit and is fed through the hole. Then it is connected to a smaller rectangular unit (the evaporator) that is attached to the wall inside of your house in the room that is to be cooled. Since there is no duct work, this is where the cool air will come from. All operating controls are on this unit or on the remote control. These units will not cool your entire home. They where mostly designed for one or two room cooling and for cooling large open spaces.