A lipoma is a benign tumour of mature fat cells in a thin capsule, sitting between the skin and the muscle. It is one of the commonest soft-tissue lumps in adults, and it is not cancer. Lipomas appear most often on the back, shoulders, neck, upper arms, chest wall and thighs, and — less often but far more visibly — on the forehead and scalp.
Lipoma or sebaceous cyst? A sebaceous (epidermoid) cyst sits within the skin rather than under it, so the skin moves with the lump instead of over it. It feels firmer, often has a small central punctum, and can turn red, tender and swollen if it gets infected. Lipomas do none of that. Other lumps mistaken for one include an enlarged lymph node, a ganglion, a dermatofibroma and an abscess, and a lump in the armpit has its own differential — axillary breast tissue is often mistaken for an armpit lipoma.
Examination settles the question in most cases. These features place a lump in the straightforward-lipoma category: