Kensho and satori are two concepts relating to enlightenment, which is one of the most intense experiences one has when practicing Zen. (The Japanese symbol for satori is included in this post). I sometimes use the two words to mean the same thing, but I think they really are somewhat different. Kensho is more about having a glimpse of your true self, while satori is a more permanent realization. Sometimes I think a kensho experience can also be satori, but not all kensho would rise to that level. It's pretty personal stuff, and can probably seem a bit simplistic to the non-practitioner.
I have had one experience during zazen that I would absolutely claim was full satori or enlightenment. In this experience, I was sitting in zazen during the spring. I had my study window open, it was early evening on a clear day, the sun was setting, and there was a fresh, gentle breeze blowing into the room. I had a candle lit on the small table I have before me when I'm seated for zazen.
I got into samadhi and was sitting in a very peaceful state, where I was not really thinking about anything. In samadhi, you just have a very acute sense of your existence at that particular moment and you aren't planning, thinking, evaluating, or using your mind in any way. As I sat, I watched the light on the table slowly lessen and the candle slowly become the only illumination on the table. I could actually watch as the light ebbed while the sun was setting, and I felt the wind gently coming in and it was if I could sense every place it blew against me. I was incredibly relaxed and happy and alert, but I was not thinking about anything. This lasted for quite a while (not exactly sure how long, but I think it must have been thirty minutes or so).
As I was coming out of samadhi, I looked up and out the window and realized that the reason the light had been ebbing was not the sunset. Dark blue-black storm clouds had silently rolled in without my realizing it. And now the candle that I had lit was glowing in the study, which was by now very dim, and the trees outside were tossing in the wind. It was clear quite a storm was brewing, but the sound of the wind in the leaves had not alerted me that this was a storm coming as opposed to the wind maybe picking up a little.
Needless to say I was completely surprised at what a transformation had occurred outside! Yet, despite all that bluster outside (it really started getting windy at this point and the thunder started rumbling ominous and low), my candle was flickering peacefully and safe. I realized that this was exactly what I was trying to achieve through Zen. The idea that me and this little candle of illumination were so calm and serene while the storm raged about outside was exactly what I sought in my everyday life: the ability to not be swept up in the world around me. The ability to be calm and centered and happy, even while everything around me became chaotic, even threatening.
This image or realization stuck with me and gave me something to think about when I felt myself getting carried away by drama or emotions or stuff that just doesn't matter (or maybe it did matter but there was no reason to allow myself to get worked up anyway). If it wasn't enlightenment, then it was as close as I have come so far. And I have certainly been much better at living life in a calmer state since then. Not that I don't still fly off the handle or get wrapped up in worries about stuff that is never even going to happen, but I have been much better able to master my mind and thoughts since this experience, which is why I think it rises to the level of satori.
Continuing to study Zen will hopefully help me enhance this control and mastery until I can exist entirely in that kind of mindspace where, no matter what is going on around me, I can maintain my sense of self. It probably all sounds very simplistic to another person (most Zen experiences do), but it was incredibly powerful for me - especially at that time in my life - and it really changed me as a person.