重油是原油提取汽油、柴油后的剩余重质油，其特点是分子量大、黏度高。重油的比重一般在0.82～0.95，比热在10,000～11,000kcal/kg左右。其成分主要是碳氢化合物，另外含有部分的（约0.1～4％）的硫黄及微量的无机化合物。主要用于大型蒸汽轮机锅炉及中大型船舶用引擎的燃油，不过由于现在大功率燃气轮机的出现，以及重油燃烧对环境的污染，现在重油锅炉几乎被淘汰了。不过，重油可以用作现代大型燃煤锅炉的启动点火燃料。现在对重油的研究方向主要是长链烃的裂化，目的是通过裂解重油得到轻质的柴油或者煤油。此外，二战其间重油可用于制造燃烧弹。

Heating oil, or oil heat, is a low viscosity, liquid petroleum product used as a fuel oil for furnaces or boilers in buildings. Home heating oil is often abbreviated as HHO.

Heating oil consists of a mixture of petroleum-derived hydrocarbons in the 14- to 20-carbon atom range that condense between 250 and 350 °C (482 and 662 °F) during oil refining. Heating oil condenses at a lower temperature than petroleum jelly, bitumen, candle wax, and lubricating oil, but at a higher temperature than kerosene, which condenses between 160–250 °C (320–482 °F). The heavy (C20+) hydrocarbons condense between 340–400 °C (644–752 °F).