真空是一种不存在任何物质的空间状态，是一种物理现象。在真空中，声波因为没有介质而无法传递，但电磁波的传递不受真空的影响。粗略地说，真空是指在一区域之内的气压远远小于大气压力。真空常用帕斯卡（Pascal）或托尔（Torr）做为压力的单位。目前在自然环境里，只有外太空堪称最接近真空的空间。

真空下的气压为零，有些情形下，气压小于大气压力，但不为零，此时称为局部真空，有些也简称为真空。

在局部真空的情形下，若其他条件不变，气压越低，表示越接近真空。例如一般的吸尘器的吸力可以使气压降低20%。也可以以产生更接近真空的条件，像化学、物理及工程常见的超高真空腔体，其气压可以到大气压力的10，粒子密度为100粒子/cm，对应约100粒子/cm。外太空更接近真空，相当于平均一立方公尺只有几个氢原子。根据现代物理学的了解，即使空间中的所有物质都移除了，因为量子涨落、暗能量、经过的γ-射线和宇宙射线、微中子等现象，空间仍然不会是完全的真空。在近代的粒子物理中，将真空态视为是物质的基态。

Vacuum is space void of matter. The word stems from the Latin adjective vacuus for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often discuss ideal test results that would occur in a perfect vacuum, which they sometimes simply call "vacuum" or free space, and use the term partial vacuum to refer to an actual imperfect vacuum as one might have in a laboratory or in space. In engineering and applied physics on the other hand, vacuum refers to any space in which the pressure is lower than atmospheric pressure. The Latin term in vacuo is used to describe an object as being in what would otherwise be a vacuum.