Tonsillar cyst or mucocele: This is a common, benign condition that can present as a smooth, round mass behind the tonsil, often without symptoms like pain, dysphagia, or odynophagia. It occurs due to the obstruction of a mucous gland in the tonsil.

Tonsillolith (tonsil stone): Although often associated with bad breath and sometimes discomfort, a tonsil stone can present as a mass behind the tonsil. It's formed from accumulated debris and can be asymptomatic.
Peritonsillar abscess: Typically presents with severe pain and difficulty swallowing, but early stages might be less symptomatic. It's an accumulation of pus behind the tonsil, usually as a complication of tonsillitis.
Lymphoid hyperplasia: An increase in the size of lymphoid tissue can occur in response to chronic inflammation or infection, presenting as a mass.

Malignant tumor (e.g., lymphoma, squamous cell carcinoma): Although less common, these conditions can present as a painless mass and are critical to diagnose early due to their potential for serious morbidity and mortality.
Infectious mononucleosis (mono): Caused by Epstein-Barr virus, it can lead to lymphoid hyperplasia and present with a mass, although typically accompanied by systemic symptoms like fever and fatigue.

Branchial cleft cyst: A congenital anomaly that can present as a smooth, painless mass in the neck, potentially near the tonsil area, though it's more commonly found along the anterior border of the sternocleidomastoid muscle.
Dermoid cyst: A rare, benign cyst that can occur in various locations, including the head and neck, potentially near the tonsils, though this would be an unusual location.