"Nosology" is the classification of diseases. Given the limited context you provide, I would suggest "diagnostic code."

no·sol·o·gy
n.
1. The branch of medicine that deals with the classification of diseases. Also called nosonomy, nosotaxy.
2. A classification of diseases.
http://medical-dictionary.thefreedictionary.com/nosology

The development of SNIPS had three stages: (1) defining categories of disorders from a specialist surgical perspective; (2) choosing a primary care terminology and classifying it using the surgical categories; and (3) employing a large national dataset of general practice activity to improve the precision of the classification by restricting the nosology to disorders that are consistently referred to surgeons.
http://www.biomedcentral.com/1472-6963/4/8

Acute transverse myelitis (ATM) is a focal inflammatory disorder of the spinal cord resulting in motor, sensory and autonomic dysfunction. We propose a set of uniform diagnostic criteria and nosology for ATM to avoid the confusion that inevitably results when investigators employ differing criteria.
http://www.myelitis.org/Diagnostic criteria.htm

Hence we developed the Surgical Nosology In Primary-care Settings (SNIPS). We have used the classification system in analysis of a national dataset of general practice activity to describe referrals to surgeons [9] and to study the influence of specialist proximity on referral rates [10].
http://ukpmc.ac.uk/articles/PMC434523/

This is a little late for this particular question, but I'm adding my answer here to set the record straight and provide a useful translation for colleagues who search this term in the future.

Here is what a doctor friend in Italy shared with me:

Nosology is the description of diseases, but in the administrative language the "numero nosologico" identifies a hospital record number.

" ... all’atto dell’accettazione viene generato un numero identificativo del ricovero (numero nosologico),... "

Il nosologico è il numero della SDO (scheda di dimissione ospedaliera) e del ricovero; viene assegnato automaticamente dal programma. Nel’attuale configurazione del programma ADT nosologico = numero della SDO = numero del ricovero.