Erythema ab igne is a skin reaction caused by exposure to heat. It is also known as hot water bottle rash, fire stains, and toasted skin syndrome. Ignis is the latin word for "fire".

In this case report a 12-year-old boy presented with erythema ab igne on his left thigh caused by the use of a laptop computer. This is the youngest of the 10 reported patients with this laptop-induced dermatosis since its first description in 2004.

Erythema ab igne is a reticular, pigmented, sometimes telangiectatic dermatosis that is caused by prolonged exposure to a heat or infrared source (see pictures from different cases at DermNet).

In laptop-induced erythema ab igne, the localization on the thighs and asymmetry are characteristic. The heat originates from the optical drive, the battery, or the ventilation fan of the computer.

References:
Laptop Computer–Induced Erythema ab Igne in a Child and Review of the Literature. PEDIATRICS Vol. 126 No. 5 November 2010, pp. e1227-e1230 (doi:10.1542/peds.2010-1390)
Erythema Ab Igne. eMedicine Specialties > Dermatology > Environmental.
Image explanation:  I was not able to find online photos of erythema ab igne labeled for reuse, hence the image of a laptop from Amazon.com.