A P45 is issued by the employer when an employee leaves. It is a multipart form. In the UK, the front section, Part 1, is given by the employer to Her Majesty's Revenue and Customs, who then record the pay and tax details on to the individual's taxpayer record. Part 1A is to be retained by the employee, Part 2 retained by the new employer, and Part 3 taken by the new employer and sent to their tax office. The P45 contains details of earnings and tax paid during the tax year (tax paid in previous years is detailed on the P60 for that year).
The "P" code refers to documents in the PAYE series, in the same way that self-assessment documents are prefixed "SA" (e.g. SA100 - Individual tax return) and Tax Credits paperwork is prefixed "TC" (e.g. TC600 - Tax credits application).
In the Republic of Ireland, P45 is also used as a reference to a form used by the Office of the Revenue Commissioners with the same function. The administrative procedures in this area are similar; however, the form itself is different in design from the UK version.
In direct reference to the form, P45 is also the name of one of Ireland's oldest and largest online discussion forums (P45.net), for office workers.