I see these two terms are used in same context. But what is the difference in meaning? Is difference amplifier a subset of differential ampfier with unity gain? I couldnt find a duplicate question.
There is " no difference " between the terms "Difference Amplifier" and "Differential Amplifier" (pun intended)
They both amplify the difference between 2 nodes, whether you have a signal on both or just one and connect the other one to some DC reference voltage.
Then the impedance ratio is 50% going into the non-inverting side with a gain of 1/2*2=1, it is unity gain while the inverting side has the same divider ratio with a gain of -1 then it is a "unity-gain differential (or difference) amplifier" which is a special case of any "diff amp".( slang description = diffy)
The gain and frequency shape can be varied as long as the ratios are matched. So instead of simply Rf/Rin=gain it can be any combination of RLC parts matched on both sides.
Matching is the hard part for discrete parts (0.1% error on 4 parts < 55 dB CMRR while integrated INstrument Amps (INA) offer CMRR > 110 dB needing only 1 R shared to control gain instead of matching 4 parts or 2 ratios, using laser trimmed internal R's.