free induction decay 

Initialism: FID
https://doi.org/10.1351/goldbook.08249
Indication from a pulsed spectroscopy experiment that takes the form of damped sine waves arising from the excited system returning to equilibrium.
Note:
In nuclear magnetic resonance spectroscopy, the FID consists of sine waves oscillating at the Larmor frequency (ω) and dampened by net dephasing time relaxation (T2), sin(ωt)exp(t/T2).
Source:
PAC, 2021, 93, 647. (Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)) on page 659 [Terms] [Paper]