Plantar Fibroma: This is a common benign growth on the plantar fascia, the band of tissue that supports the arch of the foot. It often presents as a firm, painless lump in the arch.

Plantar Fasciitis: Although not typically described as a "bump," chronic inflammation of the plantar fascia can lead to thickening and a palpable area that might be perceived as a bump.
Morton's Neuroma: A condition where the tissue around a nerve leading to a toe becomes thickened, causing pain and sometimes a palpable mass, typically between the toes but can refer pain to the arch area.
Bursitis: Inflammation of a bursa (fluid-filled sac) in the foot can cause swelling and a bump, especially if it occurs in a bursa near the arch.

Gout or Pseudogout: Deposits of uric acid or calcium pyrophosphate can accumulate in the joints, including those in the foot, leading to acute inflammatory reactions and potential swelling that might be mistaken for a bump.
Bone Tumor: Although rare, tumors such as osteochondromas or other benign or malignant growths could present as a bump in the arch of the foot.
Infection (Abscess or Osteomyelitis): An infection in the soft tissue or bone of the foot could cause swelling and a palpable mass, which is a medical emergency.

Ganglion Cyst: A lump or mass that occurs under the skin, typically on the top of the foot or wrist but can occur on the bottom, filled with a jelly-like fluid.
Lipoma: A rare occurrence in the foot, lipomas are benign tumors composed of fat tissue and can appear as a soft, movable lump.
Foreign Body Reaction: A reaction to a foreign object embedded in the foot, which could cause swelling and a palpable mass.
Sesamoiditis: Inflammation of the sesamoid bones or the tendons surrounding them, located under the first metatarsal bone, could potentially cause a bump if significant swelling occurs.