1. A breaking out, especially the appearance of lesions on the skin. 2. A rapidly developing dermatosis of the skin or mucous membranes, especially when appearing as a local manifestation of one of the exanthemata; an eruption is characterized, according to the nature of the lesion, as macular, papular, vesicular, pustular, bullous, nodular, erythematous, etc. 3. Passage of a tooth through the alveolar process and perforation of the gums.