Biologists have borrowed the atomic scientist's notion of half-life (t½) to characterize rate of exponential decay. Engineers use tau (𝝉). We use Daily Rate of Loss.
The term presumes an exponential decay, normally towards zero. It equally applies to decay towards steady-state asymptotically approached.
Popular weight loss literature usually talks in linear terms, pounds per week, and uses the term "plateau" to admit that this doesn't go on forever.
We favor the exponential model. We've seen weight loss modeled as parabolic (in the Minnesota starvation study wikipedia 
) as well as polynomial.
Biological half-life used to characterize the elimination of substances from the body. wikipedia