What is hoarding?

First, I have to point that the term "hoarding" is quite delicate to use. There is a clear distinction between Hoarding Disorder (HD) and Hoarding Behavior (HB).

The Diagnostic and Statistical Manual of Mental Disorders – 5th Edition (DSM-5, American Psychiatric Association, 2013) defines Hoarding Disorder (HD) as follows:

1. Persistent difficulty discarding or parting with possessions, regardless of their actual value.
2. This difficulty is due to a perceived need to save the items and to distress associated with discarding them.
3. The difficulty discarding possessions results in the accumulation of possessions that congest and clutter active living areas and substantially compromises their intended use. If living areas are uncluttered, it is only because of the interventions of third parties (e.g., family members, cleaners, authorities).
4. The hoarding causes clinically significant distress or impairment in social, occupational, or other important areas of functioning (including maintaining a safe environment for self and others).
5. The hoarding is not attributable to another medical condition (e.g., brain injury, cerebrovascular disease, Prader-Willi syndrome).
6. The hoarding is not better explained by the symptoms of another mental disorder (e.g., obsessions in obsessive-compulsive disorder, decreased energy in major depressive disorder, delusions in schizophrenia or another psychotic disorder, cognitive deficits in major neurocognitive disorder, restricted interests in autism spectrum disorder).

The DSM-V also calls for the clinician to specify whether the individual is also experiencing “excessive acquisition” (e.g., do they acquire items that they don’t need and for which they don’t have space in their home?) and to specify the person’s level of insight (good, fair, poor, or absent/delusional).

Source: https://hoarding.iocdf.org/professionals/diagnosing-hoarding-disorder/