Period of decay, if ..... (4-4)
I believe the answer is:
half-life
'period' is the definition.
(I know that half life is a type of period)
'decay if' is the wordplay.
I cannot really understand how this works, but
'if' is within the answer.
This explanation may well be incorrect...
'of' is the link.
Can you help me to learn more?
(Other definitions for half-life that I've seen before include "Measure of radioactive decay rate" , "Time of isotope decay" , "Measure of persistent radioactivity" .)