Half-Life:The time required for any specified property (e.g., the concentration of a substance in the body) to decrease by half. Like waiting for half your chocolate bar to disappear if each bite takes away half!
Radioactive Decay:A spontaneous process by which unstable nuclei lose energy. It's like the chocolate bar losing its size with each bite.
Decay Constant: The proportionality between the decay rate and the number of atoms which have not decayed. It's like how fast you're eating that chocolate bar.