Free Induction Decay Definition:

In NMR spectroscopy, a complex signal which is a combination of all of the electrical impulses generated by each type of proton.

Free Induction Decay Explained:

The FID is produced by the macroscopic magnetization after the pulse, the magnetization will undergo several processes as it returns to equilibrium. First, immediately after the pulse, the transverse component of the macroscopic magnetization, Mxy, will begin to process at its Larmor frequency. This precessing magnetization will induce an alternating current in a coil.