"Liquid nitrogen" is a substance that is extremely cold and looks like a clear liquid. It is used in various fields like science and medicine because it can freeze things very quickly. Full definition
One leaf per tree (third or fourth fully expanded leaf from the top) was immediately frozen in liquid nitrogen for molecular analyses. (academic.oup.com)
In addition, guests were treated to ice cream the students made with the use of liquid nitrogen. (buffalolib.org)
They preferred the fact that we occasionally use liquid nitrogen to make ice cream. (science.sciencemag.org)