A population is the entire set of units from which a sample may be drawn.
A population is the entire set of units from a group which is to be studied. Therefore the definition of a population depends on the actual research question. For instance, if the interest lies in estimating electricity consumption of U.S. households, it might be too costly and/or time-consuming to collect data for all households. For this reason we usually gather a (representative) sample of this population to draw inference. Imagine we had sampled all U.S. households, then we would have data on the entire population for our research question. If the question changes to "estimate the electricity consumption in North America", our previous population data becomes a sample data again.
