https://www.bris.ac.uk/integrative-epidemiology/intranet/training/
In an epidemiological context, whether stated explicitly or not, we typically are referring to a counterfactual condition. A counterfactual condition is where we suppose that the outcome would not have happened in an alternate reality where all things were identical except for the exposure.
For example, if an individual had body mass index = 23 instead of body mass index = 27, but all else was identical, what would the impact be on their bone mineral density?