Tap

Tap is the touch primitive everything else is measured against: one finger down, one finger up, in roughly the same place. Material describes it as press and lift, and every other gesture in the vocabulary is a deviation from it, by adding a second contact, by moving before the lift, or by staying down. Because it is the default, it is also the gesture that needs no teaching and no affordance beyond looking like something you could touch, which is why a design that hides its primary action behind anything else is spending a budget it does not have.

Tap and click are the same intention on different hardware, and the sloppiness in everyday speech is mostly harmless: people say click on a phone and are understood. It matters in two places. In interface copy, name the gesture the reader is actually making, or avoid the verb entirely (“choose a plan” survives both). In code, click is not a mouse event on the web, it is the activation event, and it fires for a tap, for Enter on a focused button, and for assistive technology. Listening for touchstart to feel faster is how a control ends up firing on a scroll that started on top of it.

The gesture has a history worth knowing, because it explains an old smell. Mobile browsers used to wait about 300 milliseconds after a touch ended to see whether a second one was coming and the tap was really a double tap for zoom. That delay made every button feel slightly dead, and a generation of libraries (FastClick and its imitators) existed to work around it. Browsers eventually removed the wait for pages that declare a sensible viewport or disable double-tap zoom, so the fix today is a meta viewport tag or touch-action: manipulation, not a script.

Two constraints come with the finger. It is far less precise than a pointer, so the touch target has a floor (44 to 48 device-independent pixels, depending on whose guidance you follow) that has nothing to do with how big the icon looks. And it does not rest anywhere before it commits, so there is no hover state to preview with: whatever a mouse would have shown on the way in has to be visible already, or moved into the result of the tap itself.