Sebaceous cyst: A common, benign cyst that can appear as a bump behind the ear, often present for years without causing symptoms. The long duration of the bump (2 years) and its nontender nature support this diagnosis.

Epidermoid cyst: Similar to a sebaceous cyst but lined with epidermal cells, these cysts can also present as long-standing, nontender bumps.
Lipoma: A benign tumor composed of fat tissue, lipomas can occur almost anywhere on the body, including behind the ear, and are typically soft and nontender.
Wart: A viral infection causing a benign growth, warts can appear behind the ear and may be nontender, though they are usually smaller and more irregular than cysts or lipomas.

Malignant tumor (e.g., basal cell carcinoma, squamous cell carcinoma): Although less likely, any new or persistent skin lesion, even if nontender, should be evaluated for the possibility of skin cancer, as early detection is crucial for treatment and prognosis.
Metastatic lesion: Rarely, a bump behind the ear could be a metastasis from a cancer elsewhere in the body, emphasizing the importance of a thorough medical history and examination.

Pilomatrixoma: A rare, benign tumor of the hair follicle, pilomatrixomas can appear as firm, nontender bumps and are more common in children and young adults.
Dermatofibroma: A benign growth that can appear after minor injury, dermatofibromas are usually found on the legs but can occur elsewhere, including behind the ear, and are typically firm and nontender.
Neurofibroma: A benign tumor of the nerve tissue, neurofibromas can occur anywhere, including behind the ear, and may be associated with neurofibromatosis type 1 (NF1) if multiple lesions are present.