180° pulse

https://doi.org/10.1351/goldbook.08310
Pulsed radiofrequency signal in NMR that rotates the bulk magnetization vector (\(\boldsymbol{M}_{0}\)) of a nuclear spin to its opposing direction, effectively inverting the signal, (e.g., from the \(+z\) to \(-z\) axis).
Source:
PAC, 2021, 93, 647. (Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)) on page 681 [Terms] [Paper]