This is the fastest-moving of the four decay mechanisms and the cheapest to correct once anyone notices. It is also the one most likely to be mistaken for a positioning problem, because the symptoms are identical.
What actually happens
A business learns how its customers describe their problem, and builds its language around that. It is good work and it is correct at the time. Then the market moves: a new term arrives, an old one acquires a negative association, a category gets renamed by someone with more reach.
The business does not notice, because internally the old words still describe the thing accurately. They have simply stopped being the words the customer would use.
Why it is invisible internally
Everyone inside has watched the vocabulary evolve and it reads as current to them. Nobody inside encounters the language the way a stranger does, and after a few years nobody can. The people best placed to notice are the ones who left, and they do not explain.
What it looks like from the outside
Enquiries hold steady while their quality changes. Sales conversations start with more translation than they used to. A prospect describes their problem, the business describes the same problem in different words, and both leave the call feeling slightly out of step without either being able to say why.
Why it matters more now
When consideration happens through a system assembling an answer, language is not decoration. It is how the business gets matched to a question at all. A description in the wrong vocabulary is not a weaker match, it is frequently no match, and there is no page two for it to appear on.
How to detect it
Read your own site as though you had never heard of the company. Better, get someone outside to describe what you do in their own words, and compare. The gap between their vocabulary and yours is the drift, and it is usually larger and more specific than anyone expects.
The other reliable source is recent sales calls. The words prospects use in the first two minutes are the current vocabulary, and they are recorded somewhere nobody reads for this purpose.
What to do about it
Not a rebrand. Language drift is corrected by updating how the business describes the problem, not by changing what the business is. It is the cheapest of the four repairs and it is worth doing on a schedule rather than waiting for a decline to prompt it.