tl;dr people who say 'oh I've been a bit depressed lately' after having a bad week may or may not be suffering from the mental illness known as depression, whic

tl;dr people who say 'oh I've been a bit depressed lately' after having a bad week may or may not be suffering from the mental illness known as depression, which is characterized by long term feelings of sadness, helplessness etc. and which generally interfere with the everyday life of the sufferer. Depression in terms of the mental illness is much more than just 'feeling sad', and involves many more psychological aspects and outward impacts on the persons life.