In nuclear physics, what is a half life?

Question

No wikipedia entries please.

Answer 1

half life is the time it takes a radioactive source of N nuclei to decay into half the amount.

this time is only average as nuclear decay is random, however half life seems to be different  for every nucleus.

half life x decay constant = ln(2)   [the natural log of 2]

Answer 2

Half life is the time it takes for a given quantity of a radioactive material to decay to one half of the given quantity.