Which Greek letter represents the "Half-Life" logo?

The Greek letter lambda represents the Half-Life logo primarily because it is associated with the scientific concept of radioactive decay and the measurement of half-life. In the game, Half-Life, the story revolves around a scientific facility where a catastrophic experiment leads to the release of dangerous creatures and dimensional anomalies. The lambda symbol is frequently featured throughout the game as a symbol of resistance against the oppressive forces in the facility. Thus, the use of the lambda symbol in the Half-Life logo symbolizes the scientific background of the game's narrative and reinforces the theme of rebellion against the prevailing powers.