This is a very popular question, and from a real purist point of view, meditation can be done in one of four postures: standing, sitting, lying and walking. However, it is not quite as straightforward as that, and here’s why:
Sitting is the optimal position. It provides the perfect balance of focus and relaxation, or at least the potential for it. And yes, sitting on a chair is fine. When the body is upright, both the body and mind tend to be alert and attentive. At the same time, when we are seated, there is a certain degree of letting go and relaxation that takes place. Compare this to lying down, when most people immediately feel a little bit too relaxed and drift off to sleep, or standing, when most people feel a bit too tense after just a few minutes of standing still in one place. Walking is another thing altogether.
So generally speaking, sitting is always preferable to lying. The one exception is if we find ourselves in too much pain or discomfort to sit. In these situations, of course it is OK to lie flat on the floor as an alternative. If you decide to do this, I’d recommend placing a thin pillow under your head and bending your knees to 90 degrees so that your feet are flat on the floor. That way you’ll protect your lower back without placing any additional strain on it.