Mindfulness is simply paying attention, on purpose, without judging it

Kabat-Zinn strips mindfulness of mysticism and defines it plainly: paying attention, in a particular way, to the present moment, deliberately, and without immediately evaluating what you notice as good or bad. That's the entire technical definition — everything else in the book is elaboration or application.

The "without judging" clause is doing more work than it looks like it is. Most attention is automatically evaluative — we notice a sensation and instantly label it pleasant or unpleasant, then react to the label rather than the sensation itself. Mindfulness asks you to notice the raw experience first, before the evaluation takes over.

This definition matters because it makes the practice available anywhere, to anyone, without requiring belief in anything — it's a capacity of attention, not a philosophy you need to adopt first.

One-line takeaway: mindfulness is noticing what's actually happening before you decide what it means.