We worry, for all sorts of good reasons, about frail old people. But what do we mean by 'frail'? Here's a paper which tackled the question scientifically.
Frailty was defined as a clinical syndrome in which three or more of the following criteria were present: unintentional weight loss (10 lbs in past year), self-reported exhaustion, weakness (grip strength), slow walking speed, and low physical activity.
That's #6 of Jerry Morris' seven Uses of Epidemiology - defining syndromes. Note that frailty is not disability and it's not co-morbidity; it's a different entity - a cluster of symptoms (a syndrome) which predicts various outcomes.
Without a definition science does not progress. Here the definition of frailty has prompted some research on diet as a predictor (but note the conclusion - most diets are not diets: they're part of a lifestyle).
Of course most people, most researchers even, still use the word 'frailty' loosely, with no definition at all. We could tell the same story about 'stress'.