The white, smoke-like water vapor that follows the path of an airplane is called a contrail. On a clear summer day, you might see several contrails criss-crossing the blue sky.
The next time you see the elongated, cloud-like lines in the sky that linger long after the plane that made them has disappeared, you'll know to call them contrails. They're caused by condensing water vapor from an airplane's exhaust. The word dates from 1945, a shortened version of "condensation trail," and they're sometimes also called "vapor trails."