The plot of the specific heat capacity versus temperature
The Lambda point is the temperature below which normal fluid helium (helium I) transitions to superfluid helium II (approximately 2.17 K at 1 atmosphere). The lowest pressure lambda point is at 2.172 K, 0.0497 atm and highest pressure to achieve superfluidity, the "upper point", is at 1.76 K, 29.8 atm.
The point's name derives from the graph (pictured) that results from plotting the specific heat capacity as a function of temperature (for a given pressure in the above range, in the example shown, at 1 atmosphere), which resembles the Greek letter lambda. The specific heat capacity tends towards infinity as the temperature approaches the lambda point.