1. Physics. the time required for one half the atoms of a given amount of a radioactive substance to disintegrate.
2. Also called biological half-life. Pharmacology. the time required for the activity of a substance taken into the body to lose one half its initial effectiveness.
3. Informal. a brief period during which something flourishes before dying out.
According to John Seely Brown and Douglas Thomas:
“The half-life of a learned skill is 5-years” – this means that much of what you learned 10 years ago is obsolete and half of what you learned 5 years ago is irrelevant.