Microinteraction is a tiny, single-purpose moment in an interface: the system reacts to one action so the person knows something happened. A toggle that slides. A like that pulses. A password field that turns a requirement green as you type.
They are UI details that serve UX and usability. They are not features and not full user flows. Designer Dan Saffer described them as moments that complete one small task — trigger, rules, feedback, loops. Done well, they build trust: the product feels finished. Done badly or missing, people tap twice, wonder if checkout worked, or assume the site is broken.
A real-life example of a microinteraction
Someone hits Pay on a phone. For three seconds nothing moves. They tap again and get charged twice — or they close the tab. A spinner on the button, then a checkmark, is the microinteraction that prevents that. Same idea as a coffee shop that stamps your loyalty card in front of you: you saw the action land.
Use them to confirm, not to decorate. If the animation hides a slow checkout or a vague error, it is theater. For more examples, see Microinteractions: The Invisible Details and Small Gestures, Big Experiences.