Definition:

Smoke testing is non-exhaustive software testing, ascertaining that the most crucial functions of a program work, but not bothering with finer details. 

It verifies the major functionality at high level in order to determine if further testing is possible. The Smoke test scenarios should emphasize breadth more than depth.  All components should be touched, and every major feature should be tested briefly. If test fails, the build is returned to developers un-tested.

The term smoke testing originated in the hardware industry. The term derived from this practice: After a piece of hardware or a hardware component was changed or repaired, the equipment was simply powered up. If there was no smoke, the component passed the test.