It's exactly what the word says - skin (derma) inflammation (itis) caused by exposure to plants (phyto) and sunlight (photo). My wife has experienced it after brusing against rue in our garden (which we raise for the Black Swallowtails). Other plants capable of photosensitizing human skin are listed in the Wikipedia entry, and include wild parsnip (which we encounter frequently while hiking in our part of the Midwest), parsley, celery, lemon, and lime.
The photos above are from a report on a group of children burned after playing with lime juice.
What at first seemed to be overexposure to the sun blossomed into softball-sized blisters and second-degree burns. Her girls, Jewels, 12, and Jazmyn, 9, wound up spending several days in an intensive care unit, hooked up to morphine to manage the pain...The tricky part is that even after initial clinical resolution, the victim has to minimize exposure to sunlight because the light can cause recrudescence of the lesions even without reexposure to the sensitizer.
A neighbor had a large lime tree that grew over the fence into the backyard where the girls went swimming. They had picked some of the fruits and squeezed them out into imaginary tea cups in their play lemonade stand... She remembered the girls crushing the fruits, juice sliding down their arms, splashing their legs, hitting their faces.
Via Nothing to do with Arbroath.