The Néel temperature, TN, is the temperature at which an antiferromagnetic material becomes paramagnetic - that is, the thermal energy becomes large enough to upset the magnetic ordering within the material.
The Néel temperature is analogous to the Curie temperature, TC, for ferromagnetic materials. It is named after Louis Néel (1904-2000), who received the 1970 Nobel prize in physics for his work in the area.