Heat up Definition 1 of 3Pro Tip 1/3Common Foods Use 'heat up' for usual items like leftovers, not raw ingredients to cook. I will heat up last night's pizza for lunch. Pro Tip 2/3Quick Process 'Heat up' implies a fast way to make something hot, not long cooking times. Can you heat up the muffins for a minute? Pro Tip 3/3Microwave Usage It's commonly used when referring to microwave heating, but not exclusively. Just heat up the stew in the microwave.