Synthy is how we describe really well matched synths. If you create a synth with 100 photos and 100 photos match together, that would be 100% synthy. The synthy % isn’t a perfect quality measurement on its own though. A 4 photo synth that is 100% synthy doesn’t make the best synth. In geek speak: it’s the probability that one can navigate between two photos selected at random, without replacement.