A Detailed Description of Onychomycosis. Onychomycosis is the true medical definition for any fungal infection that affects the fingernail or toenail. Just like any other medical word, it can be broken down to its Latin origins and can be directly translated.Onycho means nail, and mycosis means fungal infection. Detailed Description of Onychomycosis. Onychomycosis is the true medical definition for any fungal infection that affects the fingernail or toenail. Just like any other medical word, it can be broken down to its Latin origins and can be directly translated.