A milium is a small, superficial, keratin-filled cyst. Most patients present with multiple cysts and are thus diagnosed with milia.
Milia are off-white or pearly, superficial lesions that generally measure less than 3 mm in diameter . They are slightly raised above the skin surface.
With regards to the single entities, the following shall be stated:
Milia are diagnosed clinically. A small incision into the lesion allows for the expression of keratinous material, which confirms the established visual diagnosis . Although not usually required, tissue samples may be obtained for histopathological analyses. Microscopically, milia correspond to epidermal, keratin-filled cysts delimited by several layers of stratified squamous epithelium. These cysts originate in the infundibulum of the hair follicle. In the case of milia en plaque, immune cell infiltrates can be observed in close proximity to milia .
A milium is a single, superficial, keratin-filled cyst. However, few patients present with solitary lesions and thus, research papers and case reports usually refer to milia. Besides cosmetic issues, milia are benign and not known to be associated with morbidity and mortality. Although they are very common, evidence regarding their etiology and pathogenesis is scarce. This particularly applies to primary milia, which arise spontaneously. In contrast, secondary milia develop after trauma, on sunburned skin, or sites affected by dermatological conditions related to the presence of blisters and bullae. The topical application of corticosteroids has also been related to the onset of milia . Milia have been reported to develop after radiotherapy .
The following types of milia may be distinguished:
Of note, Epstein pearls and Bohn's nodules, i.e., gingival and palatal cysts, display certain similarities to milia and have repeatedly been referred to as the oral counterparts of the cutaneous condition. However, Spanish researchers could not confirm the association between the respective disorders  and they should be treated as individual entities .