Form validation is the check that a form’s answers are complete and acceptable before you save them. A required email left blank, a phone number with letters, a date in the past when you need a future appointment — the form should say what is wrong and how to fix it, next to the field.
It is not the whole error state. An error state is any failed moment: a declined card, a timeout, a 404 page. Validation is specifically this input cannot be accepted. UX writing writes the sentence; a microinteraction — a field outline, a checkmark — shows which row failed. Waiting until submit to list ten problems is a usability tax. At checkout, a vague “Invalid” next to the card is how carts die. A CAPTCHA asks whether a bot is filling the form. Validation asks whether the answers make sense. The CTA should name the outcome (“Book the visit”), not only “Submit.”
A real-life example of form validation
A Midtown clinic’s intake form accepts “asdf” in the phone field. The appointment is booked. The reminder text never arrives. Inline validation — “Use a 10-digit mobile number” — would have caught it before the patient left the page. A red box with no words is not validation. It is a guessing game.
Check the message, not only the color. For the form itself: 6 UX tips for web forms. For what happens to the answers after send: GET vs POST.