Anosmia is the absence of the sense of smell. It can be either partial or complete, though it is said that the complete loss of the sense of smell is fairly rare.
Causes of anosmia range from nasal polyps, exposure to toxic chemicals, complications from illnesses such as colds and sinus infections, or severe head trauma that renders the sense of smell dysfunctional.
Below is a diagram of how the sense of smell works:
Anosmia generally afflicts either the brain or a vital area of the nasal cavity and can either be temporary or permanent with regards to its cause and the severity of said cause.
It is also worthy of note that the sense of smell is paramount to the effectiveness of the sense of taste. Loss of the sense of smell drastically weakens the sense of taste, which can cause someone a chronic loss of appetite, which could lead to malnutrition and depression.