跨音速（Transonic），或称穿音速，是一个空气动力学名词，指的是一个正好在音速上下的速度范围（约0.8–1.2马赫）。其定义为临界马赫数（通常是0.8马赫附近）与一个更高速度（通常是1.2马赫）之间的速度范围，在这之间的速度范围，气流有些是超音速，也有些是亚音速。当飞行器速度超过临界马赫数，此时飞行器周遭的空气流开始有部分是超音速流，空气力学上开始出现急遽的变化，例如震波的出现；而当飞行器速度达1.2马赫时，此时所有气流皆为超音速，周遭气流变得稳定。

多数现代喷射飞机以可观的时间处在跨音速飞行。因为一个常出现在这样速度范围，称为波阻（wave drag）的效应而使这样的飞行状态显得重要。试图抵抗波阻效应的变革可在所有高速飞行器上见到；最显著的是后掠翼（swept wing）的设计，但另一个常见的形式是黄蜂腰形的机身（wasp-waist fuselage，亦称可乐瓶机身），作为Whitcomb面积律的副产品。

In aeronautics, transonic refers to the condition of flight in which a range of velocities of airflow exist surrounding and flowing past an air vehicle or an airfoil that are concurrently below, at, and above the speed of sound in the range of Mach 0.8 to 1.0, i.e. 965–1,236 km/h (600–768 mph) at sea level. This condition depends not only on the travel speed of the craft, but also on the temperature of the airflow in the vehicle's local environment. It is formally defined as the range of speeds between the critical Mach number, when some parts of the airflow over an air vehicle or airfoil are supersonic, and a higher speed, typically near Mach 1.2, when the vast majority of the airflow is supersonic. Between these speeds some of the airflow is supersonic, but a significant fraction is not.