Dynaflex was a type of 12-inch vinyl LP produced by RCA, and introduced in early 1971. It is essentially a very thin LP that was able to flex more without damage, and also claimed to produce a smoother and quieter playing surface.
When introduced, Dynaflex records had a thickness of just 0.03 inches, and a weight of 90 grams (standard LPs were usually around 135 grams).
Their thinness could cause problems with automatic record changers (two discs might drop instead of one for instance) and they needed to be played on a full-size turntable platter otherwise the unsupported section of the record might droop under the weight of the stylus.
There is debate about whether Dynaflex records sound better or worse than standard vinyl, but whatever the truth, RCA stopped using the Dynaflex name sometime in the late 1970s and returned to standard weight vinyl records.