What term refers to the expression of deep sorrow or regret?

The term that refers to the expression of deep sorrow or regret is "lament." This word captures the feeling of mourning or grief, often associated with loss or disappointment. To lament is to express sorrow publicly, whether through words, music, or other forms of expression. It signifies a profound emotional response to an unfortunate event or circumstance.

In contrast, the other terms do not convey the same sense of sorrow. "Rejoice" and "celebrate" relate to feelings of joy and happiness, emphasizing positive emotions rather than sadness. "Commemorate" typically involves remembering and honoring a person or event, which may include aspects of lament but is broader in scope and not limited to the expression of sorrow alone. Therefore, "lament" is the most fitting term for deep sorrow or regret.