A bit of paper you get when you graduate the run.
You can get replacement certificates at no cost from the Assassins' Guild secretary in the office by asking nicely.
When looking at it, you'll see:
A thick piece of parchment with a black silk ribbon border and the Assassins' Guild seal embossed on it. This certificate proves that the bearer is a graduated Assassin. Assassins are to carry it with them at all times, and it will keep a record of any and all inhumations the Assassin carries out for the Guild. You could also review the charter of rules adopted by the Guild of Assassins. It appears to have something written on it.
Reading it will produce a list of people you have inhumed:
This is to certify that <your name> has graduated from the Ankh-Morpork Guild of Assassins. Signed, <assessing teacher>. For the Guild of Assassins you have inhumed: Albert Dibbler Jack x4 Ned x4 Blithesome x4 Hoff Lag Sokkard For a total of 17 inhumes. For a grand total of: 17 inhumes.