An Etymological Dictionary of Astronomy and Astrophysics

The temperature (roughly 2.17 K) at which → liquid helium (→ helium I) becomes → superfluid (→ helium II).

See also: The name was given by the Dutch physicist Willem Hendrik Keesom (1876-1956), who discovered the behavior of helium near this transition point and successfully solidified helium in 1926 (under an external pressure of 25 atmospheres). The name was originally suggested by Paul Ehrenfest (1880-1933), who
was inspired by the shape of the → specific heat curve, which
resembles the Gk. letter → lambda; → point.