Delta-limiting filter
I guess that would be a slew rate limiter. This is concept is mostly used an amplifier design as a practical constraint of the circuit. I haven't seen it applied as a digital filter. It is certainly very non-linear and it would make a poor "smoothing filter" as it's highly dependent  on the absolute amplitude. Could you shed some light on the specific application and explain what a regular low pass filter can't do the desired smoothing job?