Glossary
Field validation
Checking an answer is well-formed before it is accepted.
Field validation checks that an answer is well-formed — an email looks like an email, a date is real, a number is in range — and gives the respondent a clear, specific message when it is not. Good validation prevents bad data at the point of entry rather than discovering it later in analysis.
Timing matters. Validate a field when the respondent leaves it (on blur), not on every keystroke, and never block typing. Inline, field-level errors with concrete guidance ("Enter a date in the future") outperform a single generic error at the top of the form.
Client-side validation is a usability convenience and nothing more. It must always be re-checked on the server against the canonical schema, because anything sent from a browser can be forged or bypassed. The browser improves the experience; the server is the only place trust is established.