Wow, unfortunately, there is currently no definition in the dictionary for this one. It's basically the term in "Autism Speak" that describes the actions and behaviors used to help oneself on the Autism Spectrum calm their nervous systems. I like to think of self-stimming as a way to "scratch" a "neurological itch".
Some common forms include:
I head-bang on my pillow every night to get to sleep. (I've been doing this since I was 3 months old!)
I like to jump on my trampoline
I swing on my hammock
I use tactile elements, such as braille, or other textured material.
I tend kick or tap my feet or fingers
I clap my hands
And there may be others, but I can't think of them at the moment.