The amount of time it takes for 1/2 of it to decay or dissapear
this is liike compound interest because it takes the same amount of time to decay 1/2 of it, regardless of the amount
example
lets say half life of element x is 1 year if yo has 32 grams, after 1 year, it decays to 16g 2 years decays to 8 3 years decays to 4 4 years decays to 2 5 years decays to 1 etc