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:

Although Half-life uses it, you need to be aware that lambda is a greek letter that existed well before Half-life. Therefore it has many meanings, with the Half-life resistance being one few people in society would recognize! It’s mostly known for wavelength when measuring waves although it’s linked to Half-life is a different scientific application (denoting the time taken for radioactive material to half inactivity). Being more biologically and chemistry focused I think of lambda bacteria phages more often but most people would associate it with the other two.

The picture you provided has the upper case and lower case versions of lambda. I would recommend your girlfriend looks at all the different meanings of lambda first before she does it as Wanted Bob posted earlier –

It’s also the symbol for the radioactive decay constant which is equal to the natural logarithm of two divided by the half-life.