Sampling frame
The actual list your sample is drawn from, which is rarely the same thing as the population you want to describe.
The frame is the operational list: the customer table, the student register, the panel's members, the followers of an account. Your population is the group your conclusion is about. The gap between the two is coverage error, and it is fixed at the moment you choose the list, not later.
Write both down before you field. “All patients seen in 2026” and “patients with an email address on file in 2026” are different frames, and the second one quietly excludes people in a way that is likely to correlate with age, income and digital access.
Define the sampling frame clearly and state how you selected people from it. That lets readers assess the assumptions behind your estimates.
Where it goes wrong
Describing conclusions in terms of the population when the frame was narrower. Report what the frame was; let the reader judge the distance.