It means “half-life”.
“Half-life” is the amount of time it takes for a radioactive substance to decay into half of its original quantity of atoms.
The symbol is a lowercased lambda letter. It has been used to represent many things, however, the most obvious connection to Half-Life is that lambda is the symbol for the half-life of a given material.
Other meanings are listed here: http://en.wikipedia.org/wiki/Lambda#Lower-case_letter_.CE.BB