This is not a new term, but you may often hear traditional systems referred to as unitary. Really, this just means that the system is a manufactured unit, instead of a field built system. However, it is often used to designate traditional HVAC systems: split systems as well as packaged units.
Mini-split systems are indeed split systems, with an outdoor condensing unit and an indoor blower-coil, which is often referred to as a head. Some common heads are high wall mount, ceiling mount, floor mount, and compact cassette – which tucks away above the finished ceiling. As the name mini-split implies, both the outdoor and indoor units are much smaller than found in traditional unitary systems. For the most part, the indoor units are designed to be mounted in the space they are conditioning, with no ductwork. For this reason, many people also refer to mini-split systems as ductless systems. However, mini-split systems are actually available with both low static and high static ducted indoor heads which are designed to be installed in a concealed area. Static refers to the static air pressure difference the blower is designed operate against while moving the air. Low static units are designed for very short, single runs – not complete duct systems. High static indoor units are designed to be used with complete duct systems. They are used more in commercial applications than residential applications.
Multi-split units are designed to be used with multiple indoor heads. The idea is to install one outdoor unit to supply from two to four indoor units. On typical multi-split units, the refrigerant line sets for all heads run back to the outdoor unit. The outdoor unit has connections for up to four indoor units. Another type of system which is often referred to as multi-split uses a small refrigerant network for up to nine heads. This type of system has one line set with branches that wye off at each indoor unit. These are really more like a small variable refrigerant flow (VRF) system.
Variable Refrigerant Flow
The term variable refrigerant flow (VRF) has two uses within the industry. The more global definition simply refers to any system which varies the flow or refrigerant to match the load. This is usually accomplished by changing the compressor speed. Truthfully, most mini-split and multi-split systems fit the global definition of variable refrigerant flow. There are now several unitary systems which incorporate variable refrigerant flow. Most of these systems use variable speed compressors to match the compressor capacity and refrigerant flow to the load.
However, variable refrigerant flow (VRF) also has a more specific meaning which refers to a specific type of system. These VRF systems distribute refrigerant via a network of refrigerant piping that supplies multiple indoor heads. Heating and cooling is distributed using refrigerant lines instead of ductwork.
One of the difficulties in studying this segment of the industry is the lack of uniform terms. For example, Daikin refers to their VRF system as VRV for variable refrigerant volume. Each manufacturer tends to have their own name for the different network components. The components used to branch off from the main refrigerant line to feed an individual head have several names, depending on the manufacturer. They are called refnet joints (Daikin), branch joints (Mitsubishi), separation tubes (Fujitsu), or Y-branch fittings (LG). Most manufacturers have boxes that take a single refrigerant flow and divide it up among several heads. Common names include branch selectors (Daikin), branch controllers (Toshiba), branch boxes (Mitsubishi), flow selectors (Toshiba), or heat recovery units (LG).
Heat Recovery Systems
Again, the term heat recovery has several meanings within the field. The global definition simply refers to any system which finds a productive use for the condenser heat, such as heating hot water, or dehumidification reheat. In the VRF world, heat recovery systems refer specifically to a VRF system which can heat and cool simultaneously. This is accomplished by moving heat from rooms which need to be cooled to rooms which need to be heated. This way the heat is recovered and reused, rather than being discarded.
Hopefully this discussion will help you make sense of the mini-multi jargon. For more details, refer to Unit 46 Mini-Split, Multi-Split, and VRF Systems in Fundamentals of HVACR, 3rd edition.