It seems that currently the term is more a marketing term than an engineering term.
When fears arose among airline officials that the word “turboprop” would meet with consumer resistance, the term “prop-fan” was used in a poll of United Airlines passengers. It worked: 50 percent of the respondents said they’d fly on a prop-fan-powered airliner.
From Air&Space Magazine
It depends which particular features you consider an essential defining element for the term "propfan".
Many features of propfans are also found on some powerplants described as turboprops by their makers.
Contra-rotating blades? - Found on, for example the 1950's NK-12 turboprop.
Supersonic tip speeds? - NK-12 turboprop again.
More than four blades?
- The 1945 RB.50 tuboprop had five.
- The 1984 PW127 turboprop drove a six-bladed prop.
- The 2009 TP400 turboprop drives an eight-bladed propeller.
- Curved blades? - TP400 turboprop.
- No Reduction Gearbox - A key distinction of a key forerunner, the GE36, was the lack of a reduction gearbox as used in turboprops.
- GE called their GE36 an unducted fan (UDF).
- Rolls Royce call their design an open rotor engine.
- The acronym CROR is also in used; it stands for Contra-Rotating Open Rotor.
The marketing department of its designers call it a propfan.
- Turbofan: A jet engine in which a turbine-driven fan provides additional thrust.
- Turboprop: A jet engine in which a turbine is used to drive a propeller.
- Propfan: No results found for “propfan”.
Mirriam Webster - same as Oxford essentially.
Aviation terms.com also defines turboprop etc but not propfan.
Aviation Canada, and other aviation glossaries I looked at, also fail to mention propfan.
propfan. Aeronautics. a turbojet having a turbine-driven propeller that operates completely outside the jet engine.
But that definition could equally apply to any turboprop.
propfan (pl. propfans)
A modified turbofan engine with the propeller outside the engine duct of an aircraft.
Which is not any better.
Wikipedia tries to define propfan in terms of what it isn't. The nearest it gets to a definition is to say that a propfan has a propellor with many short twisted blades. However the TP400 has many twisted blades - perhaps they are not short enough to qualify?
There doesn't currently seem to be a clear and short widely-agreed or widely-used set of engineering criteria for labeling a particular engine a propfan rather than an advanced turboprop, an unducted turbofan, an open rotor engine or any other name.
The D27 is a propfan because its maker's marketing department calls it that.