Benign neck cyst or abscess: This is often the most common cause of a lateral cervical swelling, especially if it's a new onset and there are no other significant symptoms. These can be congenital, such as branchial cleft cysts, or acquired due to infection.

Lymphadenopathy: Enlargement of the lymph nodes in the neck can cause swelling and is often due to infection, but can also be a sign of autoimmune diseases or malignancies.
Thyroid nodules or goiter: Although these typically present in the midline, large thyroid nodules or a significantly enlarged thyroid gland can sometimes cause lateral cervical swelling.
Salivary gland disorders: Disorders of the salivary glands, such as parotitis or a salivary gland stone, can cause swelling in the lateral aspect of the neck.

Malignant lymphoma: Although less common, lymphoma can present with lymphadenopathy and must be considered to ensure timely diagnosis and treatment.
Metastatic cancer: Cancers from other parts of the body can metastasize to the lymph nodes in the neck, presenting as a swelling.
Tuberculosis or other granulomatous infections: These can cause lymphadenopathy and are important to diagnose due to their public health implications and the need for specific treatment.
Carotid body tumor: A rare tumor at the bifurcation of the carotid artery, which can cause a painless lateral neck mass.

Branchial cleft anomalies: While not the rarest, these congenital anomalies can present later in life and cause lateral neck swellings.
Cervical thymic cysts: These are rare congenital anomalies that can present as a lateral neck mass.
Lipomas or other soft tissue tumors: Benign tumors of the soft tissues in the neck can cause swellings but are less common causes of lateral cervical swelling.
Castleman disease: A rare disorder that can cause lymphadenopathy and may present with a neck mass.