Is depression a mental illness?

"You don't even know what it’s like to live with a mental illness the way I do—depression is a very real thing!” 

Depression is nothing to scoff at. However, in this day and age of political correctness, fear of unintentionally offending someone, and overall awareness of a widespread amount of topics, the aforementioned statement certainly begs the question: Is depression a "mental illness"? 

According to The Mental Health Foundation, “Depression is a common mental disorder that causes people to experience depressed mood, loss of interest or pleasure, feelings of guilt or low self-worth, disturbed sleep or appetite, low energy, and poor concentration.” 

So clearly not a mental illness, correct? Well, then what makes this so? What differentiates the two? 

As Psych Central puts it, “A disorder simply means something that is out of the ordinary, which depression and other mental disorders are. They are more specifically a cluster of symptoms that research has shown to correlate highly with a specific emotional state.” 

Thus, “mental disorders” or “emotional irregularities," if we follow Psych Central's logic, and other commonly thought of "mental illnesses" in popular culture are actually disorders in reality, including bipolar disorder (yes, it's in the name, but it's still often mistaken for an illness!) and anxiety. 

So what marks the difference between a disease and a disorder, mental or not?