In the Republic of Ireland and the United Kingdom, the P45 is the form that a person is handed when leaving a job. The P45 details income for social security purposes. (The two countries' respective P45s are actually different; it's just that the UK and Ireland have very similar administrative processes here.)

Use of the term "P45" depends on context. See the following examples:

to be given your P45 -- to get fired (possibly unceremoniously)

to ask for your P45 -- to quit your job (possibly unceremoniously and/or defiantly)

to be in possession of your P45 -- to be jobless (The P45 is handed to the employer when starting at the next job.)