The term 'marked by opposite extremes' can be summarized by the word 'bipolar', which is a seven-letter word. Bipolar disorder, also known as manic-depressive illness, is a mental disorder characterized by extreme shifts in mood, energy, and activity levels. People with bipolar disorder experience periods of extreme highs (mania) and lows (depression). These episodes can last for days, weeks, or months, and can have a significant impact on a person's life.