There are several answers to the question of what defines a good meditation. They’re all valid in their own ways, but not all answers are equally helpful.
A good meditation is one where you see results
The first answer would be that a good meditation is one where things happen as planned. Most meditation practices have a particular aim in mind. Some meditations are aimed at helping you become more focused. Others help you to be more mindful, so that you observe the workings of the mind. Some aim to help you be kinder or more compassionate, or appreciative, or to have emotional stability in the face of difficulties. Some meditations aim to help you change the way you see yourself and the world around you.
Any meditation you can walk away from is a good meditation.
So let’s say that you have a particular meditation in which there’s perceptible progress in some regard. Maybe you feel very loving, or you’re joyfully and effortlessly focused. Obviously you’re going to call that a “good meditation.” And that’s fine.
But the problem is that progress in meditation is a slippery concept. It’s like physical training. Let’s say you go out for a jog, and you do your usual run in the usual time. You haven’t run a personal best. You’re not suddenly fitter or faster. Does that mean it’s not a good run? Of course not.
So if we reserve the term “good meditation” for one in which we see actual progress, then we risk creating a sense of our other meditations being “not good”, “blah,” or even “bad.” And if that happens then it saps our motivation.
A good meditation is one where very unusual things happen
Sometimes when we’re getting into a groove in meditation — perhaps the mind is calming and we’re getting more absorbed in our experience as I’ve described above — we get some added bonus special effects. These can definitely be unusual and I’ve summarized and categorized some of those in an article on Odd Experiences in Meditation. There are far more of those than I could possibly describe, but some of them include things like the feeling of floating, tingling in the spine, tingling on the top of the head (or the hands, or other parts of the body), seeing swirling lights, seeing unusually clear images in your mind’s eye, having rushing energy in the body, the body vibrating, feelings of pressure in or on the head, seeing clear light, feeling a pleasant sense of rhythm, and so on, and so on.
Some of those are a little hallucinatory, and are distractions. They’re dream-like and seem to arise from mild sensory deprivation, because although we’re getting a bit calmer we haven’t yet learned how to tap into the richness of the sensations arising in the body. Paying attention to these will just take us into a semi-dreamlike state.
Some of them are actually helpful, and are more like synesthetic experiences, where the brain takes something “good” that’s happening within our practice (such as concentration, a sense of calmness, a feeling of love) and presents it to us in the form of an image, sound, or sensation. Paying attention to those will actually take us deeper into the meditation practice.
The article I linked to above should help give you a sense of how to distinguish which are which.
Again, it’s natural to want to describe meditations in which these kinds of things happen as a “good meditation.” In fact they can be peak experiences. However, people sometimes get obsessed with them in an unhelpful way. “Oh, I had a floaty experience in meditation! Does this make me special? Maybe I’m enlightened now!” Or “I have a feeling of pressure in my head when I meditate. Maybe I have cancer!”
Although some of these experiences can be uncomfortable or dream-like, they’re still good signs. Even the experiences I’ve said it’s unhelpful to pay attention to are signs that the mind is calming down. (It’s just that you need to learn to pay more attention to the actual sensory experience arising from your body.)
But the problem with using the term “good meditation” for one in which very unusual things happen is that meditations in which these things happen are, for most people, rare. That doesn’t mean that your other sits are not good.
A good meditation is one in which you have done the work
Another answer would be that a good meditation is one where you have kept trying. You’ve taken every opportunity to return your attention to the breath, no matter how distracted you have been. Or you’ve kept on saying “May you be well, may you be at ease, may you be kind to yourself and others.
Doing those things might not have seemed to make much difference. You might have been very distracted, but every time you realized that you had been distracted you’d taken your awareness back to the breath. You might have been in a bad mood, but you kept working at cultivating kindness.
The only bad meditation is the one you didn’t do.
This is a much more useful way to think of what a good meditation consists of.
While the first two types of “good meditation” are by definition rare, you can always have “good” meditations of the type where you’re simply doing the work. Thinking of “good meditations” as being those in which you do the work is a more realistic way of looking at things. In meditation you’re working to alter your mental and emotional habits. You’re training. You’re subtly changing who you are.
The important thing, perhaps, is to recognize that you’ve worked, and to actively rejoice in it. If we’re blasé about it and take the work we do for granted, then we’re missing an opportunity to motivate ourselves.
So when you’ve worked in meditation, I suggest you literally say “Yay, me!” You can even punch a fist in the air. I pretty much guarantee that you’ll feel happier as a result (caveat: some people may have some initial embarrassment, although that will pass). And if you feel happy about your meditation sits you’ll be more inclined to keep meditating.
Any meditation you do is a good meditation
Finally, I really do think that any meditation is a good meditation. In 1944, after the plane in which he was flying crashed, the American Air Force photographer Gerald R. Massie said, “Any landing you can walk away from is a good one.”
Sometimes I say that “Any meditation you can walk away from is a good meditation.” And by walk I include “crawl” (just in case your legs went to sleep).
I also say, “The only bad meditation is the one you didn’t do.”
These, I believe, are the most useful perspectives we can have on “good meditations.”
So just do it. Do the work. Congratulate yourself on doing the work, rather than taking it for granted. Don’t get too caught up in the idea of “making progress.” And don’t spend any time at all hoping for “unusual things” to happen in your meditation, and if they do happen don’t try to make them happen again. The important thing is the process, not any specific result.