A half-life is the time period it takes for half of something to decay.[1]
Each year or "term" of the United States Supreme Court results in several decisions that directly overturn precedent, many more decisions that alter precedents, and still more decisions that create new precedents on issues of first impression. After several years, the collection of Supreme Court precedents change significantly overall.
It is thus meaningful to describe and attempt to quantify how long it takes for 50% of the precedents of the United States Supreme Court, as weighted by significance and relevance, to change. If, for example, there is roughly 5% change in the body of Supreme Court law each year, as weighted by significance and relevance, then the half-life of the Supreme Court would be roughly one decade.
This term implicitly underscores the significance of filling vacancies on the Supreme Court.