There are many kinds of meditation, with different goals and with vastly varying methods. So for scientists to be able to speak about meditation in any meaningful way, they need a way of categorising them, they need a taxonomy. And it is useful also for us practitioners, to get an overview of the field and to be able to speak to each other more clearly.
Beware that there are many ways to define meditation and many ways to categorise it, this is just one of them. And as goes the saying, every model is wrong but some are useful.
And this is exactly what Jon Nash and Andrew Newberg have created in their Toward a unifying taxonomy and definition for meditation. You can read their paper or you can listen to Nash speaking about this on the Scholar Warrior podcast. If you are short on time, start at 1:15 where he applies the taxonomy to categorise three popular, different meditation methods.
The main differentiator is the target changed state of mind, which might be cognitive, affectual, or null (i.e. neither of the two; the state of emptiness).
First: Transcendental Meditation is a null directed method. You repeat a sound, a mantra, and try being aware of it - until, eventually, all the other contents of the mind have disappeared. (But you don’t concentrate on the mantra, which would lead to the cognitive state of one-pointedness, on an exclusive concentration on single entity.)
Second: Zen koan meditation is a cognitive directed method that aims to gain an insight into the nature of reality (according to Buddha: suffering, impermanence, non-self, i.e. you can find no center of ego, it has no real existence). Also Vypassana is a cognitive directed method.
Third: Tibetan loving kindness meditation is an affective directed method, you grow the feeling of compassion and loving kindness, until, after many stages, you are totally immersed in it. The method is also (self)guided, you follow a narrative (until it too disappears in the last stage).