Half-Life
Etymology
From radioactivity research in the early 20th century; Ernest Rutherford described a "half-value period" around 1907, later shortened to "half-life." A plain compound of "half" and "life," it names how long until half a sample of radioactive atoms has decayed — a constant that holds no matter how much you start with.
noun · /HAF-lyfe/
The time it takes for half of something to decay, disappear, or lose its potency.
What it names
A precise handle for things that fade exponentially rather than all at once — a drug in the bloodstream, a scandal in the news, the relevance of a skill.
Example
Outrage online has a half-life of about a day before the algorithm moves on.
Related idea
Unlike a simple expiration, a half-life implies a long tail: something never fully vanishes, it just keeps halving — a memory or toxin can linger at trace levels indefinitely.