Definitions for "National insurance number"
A unique reference number which is used to identify your national insurance contribution record and entitlement to social security benefits.
You need this as soon as you start any work. Only one number is allocated to you and you keep that same number all your life. It is unique to you and ensures the Inland Revenue correctly record NI contributions or credits to your NI account. You will need these contributions and credits when you come to claim benefit, whether it is for a short while, like Incapacity Benefit or long term, such as your Retirement Pension.
an official coded number issued by the UK Government to every UK worker