A satori experience is when you go beyond your usual frame of reference (your usual range of perception) and have a glimpse of “the beyond.” The beyond is often experienced as a vast, perhaps infinite, expanse of soft velvety darkness.
For my first satori, I was sitting in meditation, watching my thoughts feelings and sensations. As usual, it was a very “localized” experience – close to the body. Then, all of a sudden, it felt as if the walls, floor and ceiling dropped away, and I was just suspended in this infinite void. I was so shocked. The fear came up quickly, and pulled me back.
One terminology note: satori is a glimpse of the beyond. Samadhi is a total dissolving into formlessness.