Inline validation
Also called: Live validation, Real-time validation
Showing a field's error right next to it as the user fills in the form, instead of only after submit.
Definition
Inline validation checks a field when the user leaves it (on blur) or while typing, and shows the message next to that field. Done well, it waits until the user has finished and clears the error as soon as the value is fixed. Done badly, it shouts "invalid" after the first letter.
In practice
After leaving the postcode field with "SW1A", the user sees "Enter a full postcode, like SW1A 1AA" right under it, and the message disappears once they type the rest.