Asking a lot of strangers a loaded question like this you're bound to get some iffy answers.

Let's consult with some famous faces in the community. I've provided some short excerpts so that you can skim each link and get a flavor of what the essays are like.

Thanissaro Bhikku, on mindfulness defined: https://www.accesstoinsight.org/lib/authors/thanissaro/mindf...

> Mindfulness is what keeps the perspective of appropriate attention in mind. [Goes on to discuss what different definitions of mindfulness, and where some get it wrong]

Hsuan Hua, on what meditation is: http://www.cttbusa.org/chan/chan1.asp.html#whatischan

> Chan means stilling thoughts. We will only realize Chan if we still our thoughts. Samadhi means not moving. If we move, we have no samadhi. [Goes on to discuss samadhi and posture]

Ajahn Sucitto, on different definitions of mindfulness: https://ajahnsucitto.org/articles/what-is-mindfulness/

> [Mindfulness] has a referential quality; it connects present-moment experience to a frame of reference. The teachings on the four establishments, or bases, of mindfulness exemplify this.

Ajahn Sumedho, on what mindfulness is: https://buddhismnow.com/2019/03/16/mindfulness-is-an-interes...

> It does not mean just to have an idea that one has to be mindful of each step while walking on an almsround, as a kind of fixed view of mindfulness. Because that can be merely concentration. Mindfulness allows us to really notice the way it is, where we are, the time and the place.

Ajahn Chah, on what meditation is: https://www.ajahnchah.org/book/On_Meditation1.php

> When the mind is peaceful and established firmly in mindfulness and self-awareness, there will be no doubt concerning the various phenomena which we encounter. The mind will truly be beyond the hindrances. We will clearly know everything which arises in the mind as it is. We do not doubt because the mind is clear and bright. The mind which reaches samādhi is like this.

Culadasa, on what mindfulness is: https://culadasa.com/wp-content/uploads/2021/09/Magic-of-Min...

> But what is mindfulness? “Mindfulness” is an unfortunate translation of the Pali word sati – unfortunate in that it suggests attentiveness or remembering to pay attention. This fails to capture the full meaning and importance of sati. We are always paying attention to something, even without sati, but sati means, among other things, paying attention to the right things in a skillful way. A more appropriate but clumsy phrase might be “powerfully conscious awareness”
or “fully conscious awareness”.

Bhante Vimalaramsi, on the four foundations of mindfulness: https://www.lionsroar.com/living-with-awareness-an-excerpt-f...

> The practice of contemplating (or as we might say, meditating on) the Four Foundations—mindfulness of the body, feelings, mind, and dhammas (or phenomena)—is recommended for people at every stage of the spiritual path.

Ayya Khema, on the meditative mind: https://www.vipassana.org/meditation/khema/hereandnow/medita...

> Another facet which goes together with mindfulness, is clear comprehension. Mindfulness is knowing only, without any discriminating faculty. Mindfulness does not evaluate of judge but pays full attention.

Sekkei Harada, on peace of mind: https://buddhismnow.com/2013/12/21/true-peace-of-mind-by-har...

> The expression ‘casting off body and mind’ means that by being one with a certain con­dition or circumstance, you assimilate all other things. It is said that for three years Dogen Zenji didn’t lie down to sleep and that he couldn’t recognise the faces of the monks who sat next to him. These words ‘casting off body and mind’ express the result of his ‘just sitting’. The other half of the above expression, ‘body and mind cast off’, refers to a condition where having forgotten the ego-self, everything disappears. This is all the splendid result of his having sat zazen single-mindedly.