A basic (base-forming) oxide is a metal oxide that forms hydroxide ions (OH⁻) when it reacts with water, or that is neutralised by acids. Such oxides behave as bases and are typical of metals on the left of the periodic table. Calcium oxide is a basic oxide; it contrasts with an acidic oxide, which is formed by a non-metal.

碱性氧化物是与水反应生成氢氧根离子(OH⁻)、或能被酸中和的金属氧化物。这类氧化物表现为碱,是周期表左侧金属的典型氧化物。氧化钙是碱性氧化物;它与由非金属形成的酸性氧化物相对。