A credit check from a lender that can briefly lower your credit score.
A hard inquiry happens when a lender pulls your credit report to evaluate a new application, and it can shave a few points off your score for several months. Multiple inquiries for the same purpose — such as shopping for a mortgage — within a short window are usually counted as one. A soft inquiry, like checking your own score, has no effect.
Rate-shopping mortgages within a 45-day window typically counts as a single hard inquiry.