What is the definition or description of: rash in armpit?

Better question?: The question does not really make sense and is too non-specific! if you want to know what causes the rash under the arm, it could come from deodorants more often as a contact allergy or non-specific reaction. Fungal infections and bacterial infections of the glands under the arm are to be ruled out.

Many causes: There are many causes of an armpit rash. These include a fungal infection, an allergic reaction and an infection of the hair follicle.