90° pulse 

https://doi.org/10.1351/goldbook.08311
Pulsed radiofrequency signal that rotates the bulk magnetization vector (\(\boldsymbol{M}_{0}\)) of a nuclear spin to its orthogonal plane.
Note:
In a simple 1D NMR experiment, this is from the \(z\) axis to the \(xy\) plane.
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 681 (https://doi.org/10.1515/pac-2019-0203)