Solarization is overexposure to the point of tonal reversal, where areas on the film that would be totally black (the sun, for example) start to get lighter again. You can achieve the same thing printing in the darkroom by turning on the lights during developing. (Technically doing it in camera is called soliarization and in the darkroom it’s the Sabatier Effect, but the terms are used more or less interchangeably.)
I thought that was it, until Tom Persinger of f295 pointed me to this article by Ed Buffaloe at Freestyle—it turns out you can do it chemically with thiosulfate developer.
Includes recipe and examples.