Student loan delinquency happens when you miss a payment. If your account is delinquent for more than 90 days, it will be reported to the three main credit bureaus and potentially lower your credit score.
Student loan default happens when you are delinquent for a long period of time (typically 270 days). Once you default on your loans there are serious consequences and the entire unpaid balance (plus interest) immediately becomes due. You’ll also lose eligibility for certain types of relief, such as deferment and forbearance, and your wages may be garnished. Your tax refund may also be withheld and applied to your unpaid balance.
Be sure to check out “How to Get Student Loans Out of Default: The Definitive Guide” if you’re currently in default.