The word "preheated" refers to something that has been heated to a specific temperature beforehand. There are a variety of synonyms that can be used to convey the same meaning as preheated, depending on the context. Some possible alternatives include warmed, heated up, fired up, and preparatory heated. These words are all focused on the idea of warming something up before it is used, whether that be an oven, a skillet, or some other preheating device. By using any of these synonyms in place of preheated, you can add variety to your writing and make it more interesting to read.