Onychomycosis: Fungus infection of the nail bed under the fingernails or toenails. Onychomycosis makes the nails look white and opaque, thickened, and brittle. It usually produces no symptoms other than a cosmetic problem. Treatment includes avoiding artificial nails, using safer application techniques and only new artificial nails, and using topical and oral antifungal medications. Also known as nail fungus and tinea unguium.