Definition
A check of your credit report that happens when you apply for new credit; it can lower your score slightly for a short time.
What it means
When you apply for a credit card, loan or sometimes an apartment, the lender pulls your credit report, which creates a hard inquiry. Hard inquiries stay on your report for about two years, and their small effect on scores usually fades within months. A soft inquiry, such as checking your own report, a prequalification offer or an account review by a current lender, does not affect your score. Scoring models generally treat several mortgage or auto loan inquiries made within a short shopping window as a single inquiry, so you can compare rates; the CFPB notes the window can range from about 14 to 45 days depending on the model. Many applications for different credit cards in a short time can lower a score more. You can see inquiries in your free credit reports.
Example
Shopping three mortgage lenders over 10 days usually counts like one inquiry for scoring. Applying for three new credit cards in the same 10 days counts as three separate inquiries.
Illustrative numbers only. Your situation will differ.