水平对卧引擎（英语：flat engine）乃是根据复数的汽缸分成两等分，以曲轴为中心呈现180度夹角而分列左右的形状，因而得名「水平对卧」。再者，左右对称的活塞作动时像拳击手出拳，故又名boxer engine。这个概念最早由德国机械工程师卡尔·本茨（也就是梅赛德斯-奔驰创始人之一）提出，并于1896年获得专利，如今此行种型式之引擎广被德国保时捷及日本速霸陆两大车厂所采用。

水平对卧引擎很容易跟相对活塞式引擎（opposed-piston engine）混淆，另外一种容易混淆的例子是直列型二、三、四甚至多个汽缸的引擎，倾斜呈V型180度，但一般认为这与传统直列型引擎并无二致。

A flat engine is an internal combustion engine with horizontally-opposed pistons. Typically, the layout has cylinders arranged in two banks on either side of a single crankshaft and is otherwise known as the boxer, or horizontally-opposed engine. The concept was patented in 1896 by engineer Karl Benz, who called it the "contra engine."