In Zen Training: Methods and Philosophy, Katsuki Sekida discusses four categories of "samadhi." Samadhi is one of those terms that gets used in different ways by different authors, and this often creates difficulty when discussing the concept. In the modern Christian contemplative tradition, True Self / False Self language shares this problem. Sekida uses the term in a very general way in which it may simply be taken to mean "mental absorption." He then delineates between types of mental absorptions, or "types of samadhi."
I find huge overlap in Sekida's descriptions and what I have experienced in my own practice of Centering Prayer. More than any other Zen author, I feel like his language really speaks to my own, non-officially-Zen, experience.
Sekida's categories of samadhi are:
1. Man is deprived; circumstances are not deprived.
2. Circumstances are deprived; man is not deprived.
3. Both man and circumstances are deprived.
4. Neither man nor circumstances are deprived.
I'll take each of the categories in separate posts. A preliminary note – when Sekida uses the word "man" here, he is talking about what he calls "a certain self-ruling spiritual power (which) dominates the mind." This "man" is developed through Zen practice and thus is difficult to understand for those who haven't had the experience. Here's how Sekida describes the first category of samadhi: "Man is deprived; circumstances are not deprived":
"The first category 'Man is deprived; circumstances are not deprived' denotes a situation in which one's mind is absorbed in outward circumstances. A famous surgeon was once performing an operation that required great concentration. While he was working there was a sudden earthquake. The shocks were so severe that most of the attendants involuntarily ran out of the room for safety. But the surgeon was so absorbed in the operation that he did not feel the shocks at all. After the operation was over he was told of the earthquake, and this was the first he knew of it. He had been completely absorbed in his work, in a kind of samadhi.
We experience this kind of samadhi when we are watching a football game, reading, writing, thinking, fishing, looking at pictures, talking about the weather, or even stretching out a hand to open the door – in the moment of sitting down or stepping forward. In fact, we are at every moment absorbed in that moment's action or thought. There are various degrees of absorption, various periods of duration, and differences between voluntary and involuntary attention: the differences, for example, between our watching a football game (involuntary attention) and the surgeon performing the operation (voluntary attention). But we are almost always experiencing a minor or major condition of momentary samadhi, so to speak. When we are in this sort of samadhi we are quite forgetful of ourselves. We are not self-conscious about our behavior, emotions, or thought. The inner man is forgotten and outer circumstances occupy our whole attention. To put it another way: inward concern is absent; outward concern dominates...
Now, it is important to recognize the difference between true samadhi with self-mastery and the false kind of samadhi without it. In the first, even when the inner man is forgotten, he is not forsaken. The firmly established man is getting along well within, ready to make his appearance at any time. False samadhi lacks this self-mastery from the outset. There can be fighting samadhi, stealing samadhi, hating samadhi, jealousy samadhi, worrying, dreading, upsetting samadhi, but all without the guidance of self-mastery. These are not true samadhi as it is understood in Zen...
Not losing self-mastery but at the same time being involved in external conditions is the real meaning of 'Man is deprived; circumstances are not deprived.' In this state the inner man is simply inactive."
So, according to Sekida, full absorption in what one is doing, when the inner man is "ready to make his appearance at any time," is this first kind of samadhi. Another way you could put it is that this kind of samadhi is absorption in the task at hand as directed by the "inner man" achieved through Zen practice.
This full absorption in the task at hand is, it seems to me, what Mihaly Csikszentmihalyi calls "Flow."