Lipoma: A benign tumor composed of fat tissue, which is common in adults and can appear as a superficial, non-tender lump almost anywhere on the body, including the lower back.

Sebaceous cyst: A benign, usually non-tender cyst that can appear on the skin, often on the back, and is filled with a cheesy, semi-solid material.
Epidermoid cyst: Similar to a sebaceous cyst but derived from epidermal cells, these cysts can also present as superficial, non-tender lumps.
Fatty necrosis: A condition where fat tissue under the skin dies, often due to injury, which can form a lump. It might not be immediately tender but could become so over time.
Dermatofibroma: A small, hard bump that typically develops following a minor injury, most often on the legs but can appear elsewhere, including the back.

Metastatic tumor: Although rare, a lump on the back could potentially be a metastasis from another primary cancer. This would be particularly concerning in patients with a known history of cancer.
Soft tissue sarcoma: A rare type of cancer that begins in the soft tissues, such as fat, muscle, or blood vessels. Early detection is crucial for treatment.
Abscess or infected cyst: While the lump is described as non-tender, an abscess or infected cyst could initially present this way before becoming more painful and inflamed.

Pilomatrixoma: A rare, benign skin tumor that originates from the hair follicle matrix and can appear as a lump on the skin.
Hibernoma: A rare, benign tumor of brown fat, which can present as a lump under the skin, typically in the neck, back, or thigh areas.
Neurofibroma: A benign tumor of the nerve tissue, which can appear as a lump under the skin. It's more common in individuals with neurofibromatosis type 1 (NF1), but can occur sporadically.