The short answer
Your notice period is the time between giving notice and leaving your current job. On an application, an employer usually asks because it wants to understand when you could start.
Notice period and start date are related, not identical
If you need to work two weeks after resigning, that is a notice period. If you also need time to relocate, your earliest start date may be later. Read the question closely: 'What is your notice period?' and 'When can you start?' can require different answers.
Check your employment agreement and applicable local requirements before promising a date. Notice expectations vary by employer and jurisdiction. A familiar convention is not a universal legal obligation, and an application is not the place to guess what your contract says.
Example answers for common situations
Use a duration when the form requests a duration, and a calendar date when it requests a date. For a dropdown, choose the closest accurate option and clarify separately if the employer gives you a text field.
Employed with a known notice period
Two weeks after accepting an offer and giving notice to my current employer.
Available now
No current notice period. Available to start immediately, subject to completing your onboarding requirements.
Finishing a fixed commitment
My current contract ends December 18. My earliest available start date is January 4.
Graduating soon
I finish my course requirements in May and can start June 1. I am available to interview before then.
What to enter in a required numeric box
Check whether the unit is days, weeks, or months. Two weeks entered as '2' in a days field is a different answer. Use zero for no notice only if that is accurate and the field permits it. Do not use zero simply because you cannot type an explanation.
If a form has no suitable option, use an employer contact or an available note to explain the mismatch. Keep your profile and the application consistent so a recruiter does not receive conflicting start dates.
Avoid committing before you know the terms
An availability answer is an estimate the employer can use for planning. Before resigning, understand the written offer, contingencies, and agreed start date. If your circumstances change during the process, update the recruiter promptly.
Keep a short record of the date you gave each employer. This is especially useful when several applications progress at different speeds and a date that was accurate last month has changed.
Sources and further reading
Provider and employer requirements can change. Follow the instructions for the particular application or integration.