Rewarm is a verb that describes the process of heating or reheating something that has already been heated before. There are a variety of synonyms for this word, including reheat, warm up, heat again, and warm over. Other possible synonyms include refry, reboil, recook, and reheat in the oven. Sometimes, the context of the situation will dictate which synonym is best suited for the situation. For instance, if you're reheating a bowl of soup in the microwave, you might opt for the word "reheat." On the other hand, if you're warming a pizza slice in the oven, you might use the phrase "heat again".