What is, formally speaking, major depressive disorder?
The ICD and DSM definitions can be a bit opaque, but there are several criteria that are usually considered necessary for diagnosis. Different practitioners may diagnose more conservatively or more pervasively, but those differences are based on their professional experience, and they are all meant to interpret from similar guidelines. Because there are different severities of depression, as with any illness, diagnosis relies also on the subjective judgement of a doctor or other mental health practitioner. 

Generally, if one experiences a persistently low mood and low energy for a period of longer than two weeks, they may be diagnosed with so-called "clinical" depression. Usually this entails disruption to one or more aspects of daily living, e.g. food, sleep, interpersonal relationships, work or hobbies. This type of disruption is sometimes termed "clinically significant distress", a fairly common criterion for mental illness. 

Prevalence is difficult to accurately assess. The NIMH reports 6.7% 12-month prevalence in the US. The CDC has a map of US states showing statistics for current depression, ranging from under 5% to as high as 15%. Other statistics exist in other countries. These data may reflect the best methodology available, but they are still fallible. Epidemiology of mental illness is a difficult field of study. 

The inventory you cite is one of several commonly used ones. It's also important to remember that a psychiatrist will not rely on self-report questionnaires for diagnosis. A doctor would conduct a thorough intake interview which takes into account many other factors. A fifteen or twenty question survey is not sufficient to definitively diagnose anyone.