Which term means to share in someone's sorrow or to empathize with them?

The term that means to share in someone's sorrow or to empathize with them is "commiserate." This verb specifically refers to the act of expressing sympathy or compassion towards someone who is experiencing grief or distress. It conveys a sense of understanding and sharing the emotional burden that another person is facing, making it a powerful expression of empathy.

In contrast, the other terms do not carry the same meaning. "Commingle" generally refers to mixing or combining different elements or substances together, which is unrelated to emotional support. "Concede" means to admit or acknowledge something, often in the context of a disagreement or competition, rather than expressing empathy. "Consummate" typically refers to bringing something to completion or perfecting it, which does not relate to sharing sorrow or emotional support. Thus, the correct choice reflects an important aspect of emotional intelligence and interpersonal relationships.