汽缸本体（cylinder block或engine block）是引擎的基本架构，上方紧接着汽缸头，而下方则接着曲轴箱；汽缸本体也是引擎下半座的主要部份。

汽缸本体是以压铸法或翻沙铸模法铸造而成，材质通常为铝或铁，因此又有了铝合金汽缸和铸铁汽缸之分。铸铁汽缸比铝合金汽缸还要重，一般认为铸铁汽缸的强度比铝合金汽缸大，尤其是在强制进气（如涡轮增压）的情况下，铸铁的材质会比较耐用，但其实这跟缸套的材质与设计才最有关系。汽缸的内衬（cylinder liner）称之为缸套，只有薄薄的一层，通常采用球墨铸铁（或称延性铸铁，ductile iron）制成，直接与活塞环接触，在汽缸本体铸造成型后，加工压入其内。也有仅用铝合金制成和热处理加工而没有缸套的汽缸，或是利用镍基复合电镀技术镀上一层薄膜来取代缸套，这俗称为陶瓷汽缸。

A cylinder block is an integrated structure comprising the cylinder(s) of a reciprocating engine and often some or all of their associated surrounding structures (coolant passages, intake and exhaust passages and ports, and crankcase). The term engine block is often used synonymously with "cylinder block" (although technically distinctions can be made between en bloc cylinders as a discrete unit versus engine block designs with yet more integration that comprise the crankcase as well).