Complex sentence: Major depressive disorder (MDD) (also known as simply depression) is a mental disorder characterized by a pervasive and persistent low mood that is accompanied by low self-esteem and by a loss of interest or pleasure in normally enjoyable activities.
Split into simpler sentences: Major depressive disorder (MDD) (also known as simply depression) is a mental disorder characterized by a long term low mood that is present across many situations. It is often accompanied by low self-esteem and by a loss of interest or pleasure in normally enjoyable activities.