What is did derek cheat on meredith?

As an AI language model, I cannot tell whether fictional characters have cheated on each other or not. However, in the TV show Grey's Anatomy, Derek and Meredith's relationship has been tumultuous. They have had several ups and downs throughout the show, including infidelity on both sides. At one point, Derek was emotionally involved with a nurse, Rose, while he was still married to Meredith. Meredith also had a brief flirtation with another doctor, Nathan Riggs, while she was still mourning Derek's death. Nonetheless, Derek and Meredith were ultimately each other's great loves and had a strong bond till Derek's tragic death.